[clang] [flang] [flang] Add -rtlib flag (PR #99058)

[David Truby via cfe-commits]

DavidTruby wrote:

It looks like clang_rt.crtbegin and clang_rt.crtend aren't always used on every platform so I've just removed the check for those. The check for `libclang_rt.builtins` and NOT `libgcc` and `libgcc_s` should be enough to check we are linking the right thing.

https://github.com/llvm/llvm-project/pull/99058