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

This reverts commit 0adff0af20c7d9bae8bd8bdf402506c10369ead1.

Breaks the GPU build

Add a `clock_gettime` emulation layer and use it to implement the `time`
entrypoint.

For windows, the monotonic clock is emulated using `QPC`.
The realtime clock is emulated using `GetSystemTimePreciseAsFileTime`.

`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.

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.

Reverts llvm/llvm-project#91657 and Relands #91485