[clang] [MinGW] Add the compiler rt libdirs to the search path

This matches what is done for MSVC in
b8000c0c. Since that commit, compiler
rt sanitizer libraries aren't linked to with absolute path on windows,
but using their basenames, requiring the libdirs to be passed to
the linker.

This fixes undefined behaviour sanitizer on MinGW after
b8000c0c.

Differential Revision: https://reviews.llvm.org/D79076