[PATCH][s390]: Fix the usage of store_bit_field in the backend.

[PATCH][s390]: Fix the usage of store_bit_field in the backend.

Hi All,

I seem to have broken the s390 bootstrap because I added a new parameter to the
store_bit_field function to indicate whether the value the field of is being set
is currently undefined.

If it's undefined we use a subreg instead.  In this case the value of false
restores the old behavior.

Ok for master?

Thanks,
Tamar

gcc/ChangeLog:

	* config/s390/s390.cc (s390_expand_atomic): Pass false to store_bit_field to
	indicate that the value is not undefined.