[AArch64] Allow label arithmetic with add/sub/cmp

Allow instructions such as 'cmp w0, #(end - start)' by folding the
expression into a constant. For ELF, we fold only if the symbols are in
the same section. For MachO, we fold if the expression contains only
symbols that are not linker visible.

Fixes https://llvm.org/bugs/show_bug.cgi?id=18920

Differential Revision: https://reviews.llvm.org/D23834

llvm-svn: 283862