[msan] Run the tests for each supported arch and suffix each one to distinguish them.

Summary:
Previously, the tests only ran for the 64-bit equivalent of the default target
(see -m64).
Given the supported architecture list only contains 64-bit targets, this happens
to work out the same as the supported targets in most cases but may matter for
X86_64/X86_64h on Darwin.

For other targets, the practical effect is that the test names contain the
architecture. This resolves some confusion when msan tests fail since their
name no longer implies that they are trying to test the default target.

Reviewers: samsonov

Subscribers: tberghammer, danalbert, srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D16855

llvm-svn: 260230