S390: Optimize builtin iconv-modules.

S390: Optimize builtin iconv-modules.

This patch introduces a s390 specific gconv_simple.c file which provides
optimized versions for z13 with vector instructions, which will be chosen at
runtime via ifunc.
The optimized conversions can convert between internal and ascii, ucs4, ucs4le,
ucs2, ucs2le.
If the build-environment lacks vector support, then iconv/gconv_simple.c
is used wihtout any change. Otherwise iconvdata/gconv_simple.c is used to create
conversion loop routines without vector instructions as fallback, if vector
instructions aren't available at runtime.

ChangeLog:

	* sysdeps/s390/multiarch/gconv_simple.c: New File.
	* sysdeps/s390/multiarch/Makefile (sysdep_routines): Add gconv_simple.