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

Summary:
The C library for GPUs provides the ability to target regular C/C++
programs by providing the C library and a file containing kernels that
call the `main` function. This is mostly used for unit tests, this patch
provides a quick way to add them without needing to know the paths. I
currently do this explicitly, but according to the libc++ contributors
we don't want to need to specify these paths manually. See the
discussion in https://github.com/llvm/llvm-project/pull/104515.

I just default to `lib/` if the target-specific one isn't found because
the linker will handle giving a reasonable error message if it's not
found. Basically the use-case looks like this.

```console
$ clang test.c --target=amdgcn-amd-amdhsa -mcpu=native -startfiles -stdlib
$ amdhsa-loader a.out
PASS!
```

0 files changed, 0 insertions, 0 deletions