libc++abi: fix some -Wunused-function warnings
libc++abi: fix some -Wunused-function warnings

is_initialized is only used in the no threads case or if on non ARM Apple
targets.  Use the preprocessor to remove the function otherwise.  NFC.

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@280286 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi