s390: Allow to explicitly omit base register operand in assembly

The base register operand B may be omitted in D(B) by coding D and in
D(L,B) by coding D(L). The index register operand X may be omitted in
D(X,B) by coding D(B) or explicitly omitted by coding D(,B). In both
cases the omitted base register operand value defaults to zero.

Allow to explicitly omit the base register operand B in D(X,B) and
D(L,B) by coding D(X,) and D(L,). Default the omitted base register
operand value to zero.

gas/
	* config/tc-s390.c: Allow to explicitly omit the base register
	operand in assembly.
	* NEWS: Mention that the base register now may be omitted on
	s390.
	* gas/testsuite/gas/s390/zarch-base-index-0.s: Update test cases
	for change to allow to explicitly omit the base register
	operand in assembly.
	* gas/testsuite/gas/s390/zarch-base-index-0.d: Likewise.
	* gas/testsuite/gas/s390/zarch-base-index-0-err.s: Likewise.
	* gas/testsuite/gas/s390/zarch-base-index-0-err.l: Likewise.
	* gas/testsuite/gas/s390/zarch-omitted-base-index.s: Likewise.
	* gas/testsuite/gas/s390/zarch-omitted-base-index.d: Likewise.
	* gas/testsuite/gas/s390/zarch-omitted-base-index-err.s:
	Likewise.
	* gas/testsuite/gas/s390/zarch-omitted-base-index-err.l:
	Likewise.

Reviewed-by: Andreas Krebbel <krebbel@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>