arm64: implement "BRK #imm16".


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14781 a5019735-40e9-0310-863c-91ae7b9d1cf9
Back out r14781, which was unintended.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14782 a5019735-40e9-0310-863c-91ae7b9d1cf9