[S390] add missing .set function for NT_S390_LAST_BREAK regset

The last breaking event address is a read-only value, the regset misses the
.set function. If a PTRACE_SETREGSET is done for NT_S390_LAST_BREAK we
get an oops due to a branch to zero:

Kernel BUG at 0000000000000002 verbose debug info unavailable
illegal operation: 0001 #1 SMP
...
Call Trace:
(<0000000000158294> ptrace_regset+0x184/0x188)
 <00000000001595b6> ptrace_request+0x37a/0x4fc
 <0000000000109a78> arch_ptrace+0x108/0x1fc
 <00000000001590d6> SyS_ptrace+0xaa/0x12c
 <00000000005c7a42> sysc_noemu+0x16/0x1c
 <000003fffd5ec10c> 0x3fffd5ec10c
Last Breaking-Event-Address:
 <0000000000158242> ptrace_regset+0x132/0x188

Add a nop .set function to prevent the branch to zero.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: stable@kernel.org