libm: Add feenableexcept and fedisableexcept to library on powerpc

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.

Reviewers: imp, emaste, kib

Reviewed By: kib

Differential Revision: https://reviews.freebsd.org/D35204

(cherry picked from commit 448c505c33cc334193590f3844406d6a74f26e2a)