Do not wrap expf and exp2f

The new generic expf and exp2f code don't need wrappers any more, they
set errno inline, so only use the wrappers on targets that need it.
(If the wrapper is needed, then the top level wrapper code is included,
otherwise empty w_exp*f.c is used to suppress the wrapper.)

A powerpc64 expf implementation includes the expf c code directly which
needed some changes.

	* sysdeps/ieee754/flt-32/e_exp2f.c (__exp2f): Define without wrapper.
	* sysdeps/ieee754/flt-32/e_expf.c (__expf): Likewise
	* sysdeps/ieee754/flt-32/w_exp2f.c: New file.
	* sysdeps/ieee754/flt-32/w_expf.c: New file.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/e_expf-ppc64.c: Update for
	the new expf code.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/w_expf.c: New file.
	* sysdeps/powerpc/powerpc64/power8/fpu/w_expf.c: New file.
	* sysdeps/m68k/m680x0/fpu/w_exp2f.c: New file.
	* sysdeps/m68k/m680x0/fpu/w_expf.c: New file.
	* sysdeps/i386/fpu/w_exp2f.c: New file.
	* sysdeps/i386/fpu/w_expf.c: New file.
	* sysdeps/i386/i686/fpu/multiarch/w_expf.c: New file.
	* sysdeps/x86_64/fpu/w_expf.c: New file.