assembler-arm64.h and assembler-arm64-inl.h have a B() function
which conflicts with the B macro in bytecode-utils.h.

Headers that leak macros can be annoying to deal with, in this case
we can't simply undef B at the end of source files that include
bytecode-utils.h because the second source file that includes
bytecode-utils.h won't see the B macro.  Let's just move this macro
into the two unittest files that include this header.

Bug: chromium:746958
Change-Id: I588b73fe81615f882a0e010c92ba187d3bc2bf25
Reviewed-on: https://chromium-review.googlesource.com/758779
Commit-Queue: Mostyn Bramley-Moore <mostynb@vewd.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49258}