Correctly find builtins library with clang-cl

When using COMPILER_RT_USE_BUILTINS_LIBRARY=ON and clang-cl there
where several places where it didn't work as expected.

First -print-libgcc-file-name has to be prefixed with /clang:

Then the regex that matched the builtins library was wrong because
the builtins library is called clang_rt.builtins_<arch>.lib
and the regex only matched libclang_rt.builtins_arch.a

With this commit you can use a runtime build on Windows with this
option enabled.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D120698