s390x/gdb: generate target.xml and handle fp/ac as coprocessors

s390x/gdb: generate target.xml and handle fp/ac as coprocessors

This patch reduces the core registers to the psw and the general purpose
registers. The fpc and ac registers are handled as coprocessors registers by gdb.
This allows to reuse the feature xml files taken from gdb without further
modification and is what other architectures do.

The target.xml is now generated and provided to the gdb client. Therefore, the
client doesn't have to guess which registers are available at which logical
register number.

Signed-off-by: David Hildenbrand <[email protected]>
Acked-by: Cornelia Huck <[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Jens Freimann <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>