[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.

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@362057 91177308-0d34-0410-b5e6-96231b3b80d8