[OpenMP] Fix the check for libpsapi for i386

check_library_exists fails for stdcall functions, because that
check doesn't include the necessary headers (and thus fails with
an undefined reference to _EnumProcessModules, when the import
library symbol actually is called _EnumProcessModules@16).

Merge the two previous checks check_include_files and
check_library_exists into one with check_c_source_compiles, and
merge the variables that indicate whether it succeeded.

Differential Revision: https://reviews.llvm.org/D96580