This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

When crosscompiling, we need to search for the linux kernel headers in the sysroot but since #97486 the linux kernel headers were always searched in /usr/include.

This patch fixes this behaviour by prepending a '=' to where we search for the kernel headers. As per the gcc/clang's documentation a '=' before the path is replaced by the sysroot.

This patch also includes a fix for rv32, that fails to compile due to a missing definition of CLOCK_REALTIME after this change.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

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

This patch changes the size of time_t to be an int64_t. This still
follows the POSIX standard which only requires time_t to be an integer.

Making time_t a 64-bit integer also fixes two cases in 32 bits platforms
that use SYS_clock_nanosleep_time64 and SYS_clock_gettime64, as the name
of these calls implies, they require a 64-bit time_t. For instance, in rv32,
the 32-bit version of these syscalls is not available.

We also follow glibc here, where time_t is still a 32-bit integer in
arm32.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D159125

The GPU has the ability to sleep for very short periods of time. We can
map this to the existing `nanosleep` utility. This patch maps the
nanosleep utility to the existing hardware instructions as best as
possible.

Depends on D159118

Reviewed By: JonChesterfield, sivachandra

Differential Revision: https://reviews.llvm.org/D159225