Tweak unconditional-branch.ll passing on any hosts, while investigating x86_64-mingw32.

Sorry for the breakage.

For now, it will fail in two ways:

  1. To fail for targeting x86_64-mingw32.
    <stdin>:131:8: note: possible intended match here
    0x30830a0100000002 3 0 1 0 0 is_stmt

  2. To fail not to find the target x86.
    llc: : error: unable to get target for 'x86_64-unknown-unknown',
      see --version and --triple.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205621 91177308-0d34-0410-b5e6-96231b3b80d8
Fix for PR18921 (LDRD/STRD part)::
Removed "GNU Assembler extension (compatibility)" definitions from ARMInstrInfo.td
Fixed ARMAsmParser::ParseInstruction GNU compatability branch, so it also works for thumb mode from now.
Added new tests.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205622 91177308-0d34-0410-b5e6-96231b3b80d8