[AsmPrinter] hoist %a output template to base class for ARM+Aarch64

Summary:
X86 is quite complicated; so I intend to leave it as is. ARM+Aarch64 do
basically the same thing (Aarch64 did not correctly handle immediates,
ARM has a test llvm/test/CodeGen/ARM/2009-04-06-AsmModifier.ll that uses
%a with an immediate) for a flag that should be target independent
anyways.

Reviewers: echristo, peter.smith

Reviewed By: echristo

Subscribers: javed.absar, eraman, kristof.beyls, hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60841

llvm-svn: 358618