s390/fpu: use lfpc instead of sfpc instruction

The only user of sfpc_safe() needs to read the new fpc register value
from memory before it is set with sfpc.

Avoid this indirection and use lfpc, which reads the new value from
memory. Also add the "fpu_" prefix to have a common name space for fpu
related inline assemblies, and provide memory access instrumentation.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>