[ARM][AsmParser] handles offset expression in parentheses

Summary:
Integrated assembler does not accept offset expressions surrounded by
parenthesis. Handle this case for GAS compability.
https://bugs.llvm.org/show_bug.cgi?id=43631

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68764

llvm-svn: 374832