mips32: protect mips32r2 instructions with a flag

Regression issue that came when mips_dirtyhelper_calculate_FCSR was added.
Inline assembly with MIPS32r2 instructions needs to be protected by flags
that disable it for non-MIPS32r2 platforms such as some Broadcom boards.

Fixes the issue: Bugzilla 325124.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2773 8f6e269a-dfd6-0310-a8e1-e2731360e62c