Summary:
This patch switches the GPU utility helpers to wrapping around the
gpuintrin.h ones with a C++ flavor.

Summary:
Currently fails to build libc++ because this is missing.

`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.

Summary:
This patch implements `clock_gettime` using the monotonic clock. This
allows users to get time elapsed at nanosecond resolution. This is
primarily to facilitate compiling the `chrono` library from `libc++`.
For this reason we provide both `CLOCK_MONOTONIC`, which we can
implement
with the GPU's global fixed-frequency clock, and `CLOCK_REALTIME` which
we cannot. The latter is provided just to make people who use this
header happy and it will always return failure.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

Summary:
I did this wrong in the first version, because `extern "C"` doesn't
imply it's extern when used directly.