Fix "Q" and "R" inline assembly template modifiers for big-endian Arm

Consider the endianness of the target when printing register names.  This is in line with the documentation at http://llvm.org/docs/LangRef.html#asm-template-argument-modifiers

Patch by Jackson Woodruff <jackson.woodruff@arm.com>

Reviewers: t.p.northover, echristo, javed.absar, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D49778

llvm-svn: 341052