[Sparc] Add .uahalf and .uaword directives

Summary:
Adding these makes it easier to assemble the output from GCC which
generates a lot of .uahalf and .uaword directives.

GAS treats .uahalf and .half the same unless the --enforce-aligned-data
flag is used. I could not find a similar flag for LLVM so it seems that
.half does not have any alignment requirement and is treated the same as
.uahalf should be. If that would change later on then the tests in
sparc-directives.s would fail due to bad alignment.

Reviewers: jyknight, asb

Reviewed By: jyknight

Subscribers: fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D47319

llvm-svn: 333372