[AMDGPU][llvm-mc] s_getreg/setreg* - hwreg - factor out strings/literals etc.

Hwreg(...) syntax implementation unified with sendmsg(...).
Common strings moved to Utils
MathExtras.h functionality utilized.
Added missing build dependency in Disassembler.

Differential Revision: http://reviews.llvm.org/D20381

llvm-svn: 270871