[CodeGen] fix alignment of JUMPTABLE_INSTS on v8M.base

Summary:
The attached test case fails with "fatal error: error in backend:
misaligned pc-relative fixup value" as the jump table is misaligned.
The EmitAlignment existed already for ARM and Thumb-1 code, but was
missing for Thumb-2.

The test checks that the fatal error disappears when generating an obj
file, as well as checking the align directive is there when producing an
asm file.


Reviewers: rengolin, grosbach, t.p.northover, jmolloy, SjoerdMeijer, samparker

Reviewed By: samparker

Subscribers: samparker, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D29650

llvm-svn: 294950