libm: Add feenableexcept and fedisableexcept to library on powerpc

[jhibbits]

Summary

These functions are missing from the library itself, and exist solely in
the header. This breaks a few ports that expect libm to have the
symbols in the library itself.

Questions on MFC-ability: Can this be MFC'd to 13.2, and how?

[kib]

You just merge it when you think that there is enough testing done in head, as usual. Or, is there some more background context I am not aware of?

lib/msun/powerpc/fenv.h

266–267

This comment should be moved down, perhaps?

[jhibbits]

I wasn't sure about adding new functions to libraries in stable/ branches, particularly to notify ports of the existence. I see now that stable/13 has the FBSD_1.7 in the top level Symbol.map anyway, so no issue.