Remove x86 mathinline.h.

After previous cleanups, the only code in the x86 bits/mathinline.h
that is relevant with current compilers is the inline of
__ieee754_atan2l that is conditional on __LIBC_INTERNAL_MATH_INLINES
(i.e. for when libm itself is being built).

This inline is something that does belong in glibc not GCC, since
__ieee754_atan2l is a purely internal function name.  This patch moves
that inline to a new sysdeps/x86/fpu/math_private.h, removing the
bits/mathinline.h header.

Note that previously the inline was only for non-SSE 32-bit x86.  That
condition does not make sense, however, for a long double function; if
it's not inlined, exactly the same x87 instruction will end up getting
used by the out-of-line function, for both 32-bit and 64-bit.  So that
condition is not retained in the new version.

Tested for x86_64 and x86.  As expected, installed stripped shared
libraries are unchanged for 32-bit x86, but installed stripped libm.so
is changed for x86_64 because calls to __ieee754_atan2l start being
inlined where previously they were out of line calls.  (The same
change to start inlining the function would presumably also apply for
32-bit built with -mfpmath=sse, but that's not a configuration I've
tested.)

	* sysdeps/x86/fpu/math_private.h: New file.
	* sysdeps/x86/fpu/bits/mathinline.h: Remove.