- perfmon headers must be avoided until perfmon is supported.
- all call-used registers including return registers must be preserved
by .mcount(), etc., not quite as in profile.h. __cyg_profile_func_*()
don't require this, but they are (mis)implemented as aliases for
.mcount(), etc. so they preserve the registers.
- i386 ifdefs related to perfmon have not been adjusted yet.
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