mips32/mips64: Avoid breakpoints in branch delay slots

Reusing parts of Chris Dearman's change in GDB to avoid placing breakpoints
in a branch delay slot.
Fixes gdbserver_tests/mcbreak for MIPS32 and MIPS64.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13451 a5019735-40e9-0310-863c-91ae7b9d1cf9