[llvm-mc] The object form of the GNU triple should be the same as the string form.

Summary:
GetTarget() may modify TripleName without also updating TheTriple.
This can lead to situations where the MCObjectStreamer has a different triple
to the rest of LLVM.

This inconsistency caused sparc-little-endian.s to pass on Windows because most
of LLVM had sparcel-pc-win32 while MCObjectStreamer had "". I believe the same
kind of thing was also true of Darwin.

Reviewers: rengolin

Reviewed By: rengolin

Subscribers: llvm-commits, rengolin, rafael

Differential Revision: http://reviews.llvm.org/D10450

llvm-svn: 239808