[runtimes] Use -Wunknown-pragmas for the pragma check

This is a follow up to r362055, we need -Wunknown-pragmas otherwise
the check is going to succeed it the pragma isn't supported.

llvm-svn: 362057
GitOrigin-RevId: f1ddf431b5dd47a6c03ed1420af69facace8cb1d
1 file changed