[OpenMP] Only use #pragma comment(lib, ...) in MSVC build configurations

MinGW build configurations don't support this pragma (unless
compiling with clang, with -fms-extensions, and linking with
lld), and at least clang warns about it.

This library does end up linked by the cmake files anyway (as
long as the check works properly).

Differential Revision: https://reviews.llvm.org/D96581