* lib/libmudflap.exp (libmudflap-init): For C++ test cases only,
import some build settings from libstdc++-v3 testsuite_flags.
* .../cfrags.exp, .../c++frags.exp, .../cthfrags.exp: Corresponding
changes to pass test language.
* mf-runtime.c (__mfu_check): Poison the cache with antidote for
quicker mode-nop handling.