[libc] Implement `timespec_get` (#116102)

`timespec_get` is C standard counterpart to POSIX `clock_gettime`. On
Linux we simply use `clock_gettime`. On baremetal we introduce a new
external API `_

[libc] Implement `timespec_get` (#116102)

`timespec_get` is C standard counterpart to POSIX `clock_gettime`. On
Linux we simply use `clock_gettime`. On baremetal we introduce a new
external API `__llvm_libc_timespec_get_utc` that should be implemented
by the vendor.

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[libc] fix clang-tidy llvm-header-guard warnings (#82679)


Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:

warning: header guard does not follow pref

[libc] fix clang-tidy llvm-header-guard warnings (#82679)


Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:

warning: header guard does not follow preferred style [llvm-header-guard]

This is because many of our header guards start with `__LLVM` rather than
`LLVM`.

To filter just these warnings:

$ ninja -k2000 libc-lint 2>&1 | grep llvm-header-guard

To automatically apply fixits:

$ find libc/src libc/include libc/test -name \*.h | \
xargs -n1 -I {} clang-tidy {} -p build/compile_commands.json \
-checks='-*,llvm-header-guard' --fix --quiet

Some manual cleanup is still necessary as headers that were missing header
guards outright will have them inserted before the license block (we prefer
them after).

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[libc] Fix 'clock()' testing on the GPU

Summary:
We should check for the GPU architectures first, since `__linux__` can
be set potentially during these compilations. Also the test needs to be
a herm

[libc] Fix 'clock()' testing on the GPU

Summary:
We should check for the GPU architectures first, since `__linux__` can
be set potentially during these compilations. Also the test needs to be
a hermetic test.

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[libc] Implement the 'clock()' function on the GPU

This patch implements the `clock()` function on the GPU. This function
is supposed to return a timestamp that can be converted into seconds
using t

[libc] Implement the 'clock()' function on the GPU

This patch implements the `clock()` function on the GPU. This function
is supposed to return a timestamp that can be converted into seconds
using the `CLOCKS_PER_SEC` macro. The GPU has a fixed frequency timer
that can be used for this purpose. However, there are some
considerations.

First is that AMDGPU does not have a statically known fixed frequency. I
know internally that the gfx10xx and gfx11xx series use a 100 MHz clock
which will probably remain for the future. Gfx9xx typically uses a 25
MHz clock except for the Vega 10 GPU. The only way to know for sure is
to look it up from the runtime. For this purpose, I elected to default
it to some known values and assign these to an exteranlly visible symbol
that can be initialized if needed. If we do not have a good guess we
just return zero.

Second is that the `CLOCKS_PER_SEC` macro only gives about a microsecond
of resolution. POSIX demands that it's 1,000,000 so it's best that we
keep with this tradition as almost all targets seem to respect this. The
reason this is important is because on the GPU we will almost assuredly
be copying the host's macro value (see the wrapper header) so we should
go with the POSIX version that's most likely to be set. (We could
probably make a warning if the included header doesn't match the
expected value).

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D159118

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[libc] Correct usage of __unix__ and __linux__

Reviewed By: michaelrj, thesamesam

Differential Revision: https://reviews.llvm.org/D153729

[libc] add clock_gettime

Add the clock_gettime syscall wrapper and tests.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D134773