S/390: Fix 16 bit pc relative relocs.

S/390: Fix 16 bit pc relative relocs.

Since the bpp instruction has been added the 16 bit wide pc relative
relocs might occur at offset 2 as well at offset 4 in an instruction.
With this patch the different adjustment is passed from
md_gather_operand to md_apply_fix via fx_pcrel_adjust field in the fix
data structure.

No regressions on s390x.

gas/ChangeLog:

2016-11-04  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/tc-s390.c (md_gather_operands): Set fx_pcrel_adjust.
	(md_apply_fix): Use/Set fx_pcrel_adjust.
	* testsuite/gas/s390/zarch-zEC12.d: Add bpp reloc test pattern.
	* testsuite/gas/s390/zarch-zEC12.s: Add bpp reloc test.