S390: Get rid of make warning: overriding recipe for target gconv-modules.

S390: Get rid of make warning: overriding recipe for target gconv-modules.

This patch introduces a way to provide an architecture dependent gconv-modules
file. Before this patch, the gconv-modules file was normally installed from
src-dir/iconvdata/gconv-modules. The S390 Makefile had overridden the
installation recipe (with a make warning) in order to install the
gconv-module-s390 file from build-dir.
The iconvdata/Makefile provides another recipe, which copies the gconv-modules
file from src to build dir, which are used by the testcases.
Thus the testcases does not use the currently build s390-modules.

This patch uses build-dir/iconvdata/gconv-modules for installation, which
is generated by concatenating src-dir/iconvdata/gconv-modules and the
architecture specific one. The latter one can be specified by setting the variable
sysdeps-gconv-modules in sysdeps/.../Makefile.

The architecture specific gconv-modules file is emitted before the common one
because these modules aren't used in all possible conversions. E.g. the converting
from INTERNAL to UTF-16 used the common UTF-16.so module instead of UTF16_UTF32_Z9.so.

This way, the s390-Makefile does not need to override the recipe for gconv-modules
and no warning is emitted anymore.
Since we no longer support empty objpfx the conditional test in iconvdata/Makefile
is removed.

ChangeLog:

	* iconvdata/Makefile ($(inst_gconvdir)/gconv-modules):
	Install file from $(objpfx)gconv-modules.
	($(objpfx)gconv-modules): Concatenate architecture specific file
	in variable sysdeps-gconv-modules and gconv-modules in src dir.
	* sysdeps/s390/gconv-modules: New file.
	* sysdeps/s390/s390-64/Makefile: ($(inst_gconvdir)/gconv-modules):
	Deleted.
	($(objpfx)gconv-modules-s390): Deleted.
	(sysdeps-gconv-modules): New variable.