S390: Fix for inadvertently setting 24-bit mode in fill_gregset

S390: Fix for inadvertently setting 24-bit mode in fill_gregset

On 64-bit S390 platforms, for programs compiled with -m31, it could
happen that GDB inadvertently cleared the inferior's 31-bit addressing
mode bit and left the inferior running in 24-bit addressing mode.  In
particular this occurred with checkpoint.exp, when the "restore"
command needed to create a new regcache copy: At the time when the
PSWM register was copied over, the addressing mode bit was taken from
the PSWA register, which was still zero since it had not been copied
yet.  And when the PSWA register was copied, the addressing mode was
not updated again.

The fix affects fill_gregset, where the bits "belonging" to each of
the PSWA and PSWM registers are now carefully separated.  The
addressing mode bit is no longer touched when writing PSWM, and --
more importantly -- it *is* written when writing PSWA.

gdb/ChangeLog:

	* s390-linux-nat.c (fill_gregset): Avoid relying on the PSWA
	register in the regcache when treating the PSWM register, and vice
	versa.