Fix compile error on s390x in shims/lock.h. s390x has a strong memory model and does not require membarrier.
Fix compile error on s390x in shims/lock.h. s390x has a strong memory model and does not require membarrier.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch