S390: Fix displaced stepping of "basr r,0"

S390: Fix displaced stepping of "basr r,0"

The BASR instruction behaves differently depending on whether the second
operand is a number from 1 to 15, or zero.  In the former case BASR jumps
to the address contained in the general register of that number, but in
the latter case no jump is performed.  GDB's displaced-stepping logic does
not distinguish these cases, although it should.

This is fixed.  In the case where no jump is performed the PC is adjusted
to point back after the original instruction.  Otherwise the PC is left
alone.

gdb/ChangeLog:

	* s390-tdep.c (s390_displaced_step_fixup): Adjust PC for a
	non-branching basr.