[runtimes] Use -Wunknown-pragmas for the pragma check
[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: 62ef012455b6b8fcec9c41ca270ebf409aae1006@362057 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libunwind,llvm-mirror/libunwind,llvm-mirror/libunwind