Fix gdb.base/sigstep.exp test for ppc

The test stops at <signal_handler called> which is the call to the handler
rather than in the handler as intended.  This patch replaces the
gdb_test "$enter_cmd to handler" with a gdb_test_multiple test.  The multiple
test looks for the stop at <signal_handler called>.  If found, the command
is issued again.  The test passes if gdb stops in the handler as expected.

(gdb) PASS: gdb.base/sigstep.exp: stepi to handler, nothing in handler, step
from handler: continue to signal
stepi
<signal handler called>
1: x/i $pc
=> 0x7ffff7f80440 <__kernel_start_sigtramp_rt64>:       bctrl
(gdb) stepi
handler (sig=551) at sigstep.c:32
32      {
1: x/i $pc
=> 0x10000097c <handler>:       addis   r2,r12,2
(gdb) PASS: gdb.base/sigstep.exp: stepi to handler, nothing in handler,
step from handler: stepi to handler

Patch has been tested on x86_64-linux and ppc64le-linux with no test failures.