[AArch64][AsmParser] Fix .arch_extension directive parsing

This patch fixes .arch_extension directive parsing to handle a wider
range of architecture extension options. The existing parser was parsing
extensions as an identifier which breaks for extensions containing a
"-", such as the "tlb-rmi" extension.

The extension is now parsed as a string. This is consistent with the
extension parsing in the .arch and .cpu directive parsing.

Patch by Cullen Rhodes (c-rhodes)

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D60118

llvm-svn: 357677