[clang] [Clang] Add a flag to include GPU startup files (PR #112025)

[Louis Dionne via cfe-commits]

ldionne wrote:

Just to clarify, I'm mostly neutral on this. A Clang reviewer should chime in and evaluate the change for what it is. It does simplify the setup of the libc++ test suite for GPU, but it's not required since we have other options to set things up.

https://github.com/llvm/llvm-project/pull/112025