Support: use internal `call_once` on PPC64le
Support: use internal `call_once` on PPC64le

Always use the internal `call_once` for PPC64le.  This is needed to
support the Swift toolchain on PPC64le.

Patch by Sarvesh Tamba!

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@354045 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm