S390: Optimize 8bit-generic iconv modules.

S390: Optimize 8bit-generic iconv modules.

This patch introduces a s390 specific 8bit-generic.c file which provides an
optimized version for z13 with translate-/vector-instructions, which will be
chosen at runtime via ifunc.
If the build-environment lacks vector support, then iconvdata/8bit-generic.c
is used wihtout any change. Otherwise iconvdata/8bit-generic.c is used to create
conversion loop routines without vector instructions as fallback, if vector
instructions aren't available at runtime.

The vector routines can only be used with charsets where the maximum UCS4 value
fits in 1 byte size. Then the hardware translate-instruction is used
to translate between up to 256 generic characters and "1 byte UCS4"
characters at once. The vector instructions are used to convert between
the "1 byte UCS4" and UCS4.

The gen-8bit.sh script in sysdeps/s390/multiarch generates the conversion
table to_ucs1. Therefore in sysdeps/s390/multiarch/Makefile is added an
override define generate-8bit-table, which is originally defined in
iconvdata/Makefile. This version calls the gen-8bit.sh in iconvdata folder
and the s390 one.

ChangeLog:

	* sysdeps/s390/multiarch/8bit-generic.c: New File.
	* sysdeps/s390/multiarch/gen-8bit.sh: New File.
	* sysdeps/s390/multiarch/Makefile (generate-8bit-table):
	New override define.
	* sysdeps/s390/multiarch/iconv/skeleton.c: Likewise.