Add a command line flag to change to using standard AsmPrinter

Specifying -hsail-asmprinter to llc will switch to using it, and
defaults to using BRIGAsmPrinter.

We by default register to the standard AsmPrinter, and if using the
default BRIGAsmPrinter, override the registration to avoid problems
from needing to share the command line option between multiple translation
units.