[ARM] Fix for branch label disassembly for Thumb

Different MCInstrAnalysis classes for arm and thumb mode, each with
their own evaluateBranch implementation. I added a test case and
fixed the coff-relocations test to use '<label>:' rather than
'<label>' in the CHECK-LABEL entries, since the ones without the
colon would match branch targets. Might be worth noticing that
llvm-objdump does not lookup the relocation and thus assigns it a
target depending on the encoded immediate which #0, so it thinks it
branches to the next instruction.

Committed on behalf of Andre Vieira (avieira).

Differential Revision: https://reviews.llvm.org/D30943

llvm-svn: 297821