ARM64: initial clang support commit.

This adds Clang support for the ARM64 backend. There are definitely
still some rough edges, so please bring up any issues you see with
this patch.

As with the LLVM commit though, we think it'll be more useful for
merging with AArch64 from within the tree.
CodeGen: Allow different RTTI emission strategies

Some ABIs and C++ libraries may make different trade-offs in how RTTI
is emitted (currently with respect to visibility and so on). This
implements one scheme, as used by ARM64.
llvm-mc: Fix build breakage caused by r205050.

When LLVM is not built with zlib, nocompression.s will test
for the error message.  But this test case will cause breakage
because the exit code is non-zero.  This commit fix this issue
by adding "not" to the command.