math: Remove fenvinline.h

Similar to string2.h (18b10de7) and string3.h (09a596cc) this
patch removes the fenvinline.h on all architectures.  Currently
only powerpc implements some optimizations.  This kind of optimization
is better implemented by the compiler (which handles the architecture
ISA transparently).

Also, for the specific optimized powerpc implementation the code is
becoming convoluted and these micro-optimization are hardly wildly
used, even more being a possible hotspot in realword cases
(non-default rounding are used only on specific cases and exception
handling are done most likely only on errors path).  Only x86
implements similar optimization (on fenv.h) also indicates that
these should no be on libc.

The math/test-fenv already covers all math/test-fenvinline tests,
so it is safe to remove it.

The powerpc fegetround optimization is moved to internal
fenv_libc.h.

The BZ#94193 [1] the corresponding GCC bug for adding replacements
for these on powerpc.

Checked on x86_64-linux-gnu and powerpc64le-linux-gnu.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94193