memmem: rearrange memmem and expand memmem-simple modules

memmem: rearrange memmem and expand memmem-simple modules

Move all functional checks to memmem-simple so that one has
a fully functional memmem by using just this module.
Restrict the memmem module to performance checks only.
Document exactly how the memmem and memmem-simple modules
relate to each other.

* m4/memmem.m4 (gl_FUNC_MEMMEM_SIMPLE, gl_FUNC_MEMMEM): Move the
empty needle check from the memmem module to memmem-simple.
Also expand the empty needle check to ensure the correct
pointer is returned, not just a non NULL pointer.
In gl_FUNC_MEMMEM, make the cross compilation check only
consider the versions with performance issues.
* doc/glibc-functions/memmem.texi: Rearrange the portability
documentation to correlate with the rearranged checks.