[libc] major refactor of startup library (#76092)

* separate initialization routines into _start and do_start for all
architectures.
* lift do_start as a separate object library to avoid code dupl

[libc] major refactor of startup library (#76092)

* separate initialization routines into _start and do_start for all
architectures.
* lift do_start as a separate object library to avoid code duplication.
* (addtionally) address the problem of building hermetic libc with
-fstack-pointer-*

The `crt1.o` is now a merged result of three components:

```
___
|___ x86_64
| |_______ start.cpp.o <- _start (loads process initial stack and aligns stack pointer)
| |_______ tls.cpp.o <- init_tls, cleanup_tls, set_thread_pointer (TLS related routines)
|___ do_start.cpp.o <- do_start (sets up global variables and invokes the main function)
```

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[libc] expose aux vector (#75806)

This patch lifts aux vector related definitions to app.h. Because
startup's refactoring is in progress, this patch still contains
duplicated changes. This problem

[libc] expose aux vector (#75806)

This patch lifts aux vector related definitions to app.h. Because
startup's refactoring is in progress, this patch still contains
duplicated changes. This problem will be addressed very soon in an
incoming patch.

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[libc] fix issues around stack protector (#74567)

If a function is declared with stack-protector, the compiler may try to
load the TLS. However, inside certain runtime functions, TLS may not be
av

[libc] fix issues around stack protector (#74567)

If a function is declared with stack-protector, the compiler may try to
load the TLS. However, inside certain runtime functions, TLS may not be
available. This patch disables stack protectors for such routines to fix
the problem.

Closes #74487.

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[libc][NFC] unify startup library's code style with the rest (#74041)

This PR unifies the startup library's code style with the rest of libc.

[libc] Mass replace enclosing namespace (#67032)

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

[libc][cleanup] Fix most conversion warnings

This patch is large, but is almost entirely just adding casts to calls
to syscall_impl. Much of the work was done programatically, with human
checking wh

[libc][cleanup] Fix most conversion warnings

This patch is large, but is almost entirely just adding casts to calls
to syscall_impl. Much of the work was done programatically, with human
checking when the syntax or types got confusing.

Reviewed By: mcgrathr

Differential Revision: https://reviews.llvm.org/D156950

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[libc][NFC] Rename files

This patch mostly renames files so it better reflects the function they declare.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D155607

[libc][NFC] Standardize missing syscalls error messages.

This patch standardizes the error messages when a syscall is not
available to be in the format: "ABC and DEF syscalls are not available."

Re

[libc][NFC] Standardize missing syscalls error messages.

This patch standardizes the error messages when a syscall is not
available to be in the format: "ABC and DEF syscalls are not available."

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D148373

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[libc] Declare __dso_handle in the integration test instead of startup.

Fixes #61355. The __dso_handle decl was introduced incorrectly into the startup
objects during the integration test cleanup wh

[libc] Declare __dso_handle in the integration test instead of startup.

Fixes #61355. The __dso_handle decl was introduced incorrectly into the startup
objects during the integration test cleanup which moved the integration tests
away from using an artificial sysroot to using -nostdlib. Having it in the
startup creates the duplicate symbol error when one does not use -nostdlib.
Since this is an integration test only problem, it is meaningful to keep it in
the integration test anyway.

Differential Revision: https://reviews.llvm.org/D145898

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[libc] Simplify integration tests by eliminating the artificial sysroot.

The test binaries are built like any other executable but with two
additional linker options -static and -nostdlib.

Reviewed

[libc] Simplify integration tests by eliminating the artificial sysroot.

The test binaries are built like any other executable but with two
additional linker options -static and -nostdlib.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D145298

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[libc][NFC] Rename "loader" to "startup".

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D140049