ARM: diagnose deprecated syntax

The use of SP and PC in the register list for stores is deprecated on ARM
(ARM ARM A.8.8.199):

  ARM deprecates the use of ARM instructions that include the SP or the PC in
  the list.

Provide a deprecation warning from the assembler in the case that the syntax is
ever seen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224319 91177308-0d34-0410-b5e6-96231b3b80d8