Add support for nest attribute to AArch64 backend

The nest attribute is currently supported on the x86 (32-bit) and x86-64
backends, but not on ARM (32-bit) or AArch64. This patch adds support for
nest to the AArch64 backend.

Register x18 is used by GCC for this purpose and hence is used here.
As discussed on the GCC mailing list the register choice is an ABI issue
and so choosing the same register as GCC means __builtin_call_with_static_chain
is compatible.

Patch by Stephen Cross.

llvm-svn: 241794