MCAsmParser: relax declaration parsing

The Linux kernel defines empty macros for compatibility with ARM UAL syntax.
The comma after the name is optional, and if present can be safely lexed.  This
improves compatibility with the GNU assembler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201474 91177308-0d34-0410-b5e6-96231b3b80d8