[ARM][AAarch64] Initial command-line support for v8.7-A

This introduces command-line support for the 'armv8.7-a' architecture name
(and an alias without the '-', as usual), and for the 'ls64' extension name.

Based on patches written by Simon Tatham.

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D91776