update build infrastructure for llvm-16.0.6

Update clang build infrastructure for LLVM 11.1.0.

Heavy lifting by mortimer@, thank you so much!

Implement Loongson 2F BTB bug workaround in clang

This adapts the -mfix-loongson2f-btb workaround from as(1) to
LLVM/clang. Because LLVM emits machine code directly without using
assembly as interme

Implement Loongson 2F BTB bug workaround in clang

This adapts the -mfix-loongson2f-btb workaround from as(1) to
LLVM/clang. Because LLVM emits machine code directly without using
assembly as intermediary form, the workaround has to be applied
in the compiler backend in addition to the integrated assembler.

Discussed with miod@
OK mortimer@

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Update build infrastructure for LLVM 10.0.0.

ok hackroom@
tested by plenty

Add retguard for octeon/mips64.

For this architecture we use separate retguard prologue and epilogue code
for static or PIC code. In the PIC case we use some additional code before
the retguard epil

Add retguard for octeon/mips64.

For this architecture we use separate retguard prologue and epilogue code
for static or PIC code. In the PIC case we use some additional code before
the retguard epilogue to recover the function start address and the GOT
pointer in order to get the per-function random cookie. Much thanks to
visa@ for suggestions and advice making it all work.

ok deraadt@ visa@

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Update clang build infrastructure for LLVM 8.0.0.

Build and install a shared libLLVM, llvm-config and llvm includes.
This is required to build the radeonsi Mesa driver.

ok patrick@

Update clang build infrastructure for LLVM 7.0.1.

Update clang build infrastructure for LLVM 5.0.0.

zap empty depends/silence empty installs.
okay millert@

Add infrastructure to build LLVM for mips64.

"check with visa@" kettenis@
"go ahead" visa@