s390/perf: add perf register support for floating-point registers

For correct unwinding of user space processes, the floating-point
register contents are required.  For example, leaf functions might
use fp registers to temporarily store the return address.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Reviewed-and-tested-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>