s390/perf: extend perf_regs support to include floating-point registers

Extend the perf register support to also export floating-point register
contents for user space tasks.  Floating-point registers might be used
in leaf functions to contain the return address.  Hence, they are required
for proper DWARF unwinding.

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>