m68k: Remove mathinline.h

This is similar to x86 (da75c1b1) and powerpc (32ea7299)
mathinline.h removal.  The required macros to build the fpu routines
are moved to mathimpl.h, while the inline optimization macros for
atan, tanh, rint, log1p, significand, trunc, floor, ceil, isinf,
finite, scalbn, isnan, scalbln, nearbyint, lrint, and sincos are removed.

The gcc bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94204 was
created to track builtin support.

Checked with a build against m68k-linux-gnu, resulting binaries
are similar with and without the patch.