[AArch64]: add 'a' inline asm operand modifier.

Summary:
This is used in the Linux kernel, and effectively just means "print an
address". This brings back r193593.

Reviewed by: Renato Golin

Reviewers: t.p.northover, rengolin, richard.barton.arm, kristof.beyls

Subscribers: aemerson, javed.absar, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D33558

llvm-svn: 303901