[gdb/testsuite] Handle SIGILL in two gdb.arch powerpc test-cases

On powerpc64le-linux, with test-case gdb.arch/powerpc-addpcis.exp I run into
SIGILL:
...
(gdb) PASS: gdb.arch/powerpc-addpcis.exp: get hexadecimal valueof "$r3"
stepi^M
^M
Program terminated with signal SIGILL, Illegal instruction.^M
The program no longer exists.^M
(gdb) PASS: gdb.arch/powerpc-addpcis.exp: set r4
...
because it's a power9 insn, and I'm running on a power8 machine.

Fix this by handling the SIGILL.  Likewise in gdb.arch/powerpc-lnia.exp.

Tested on powerpc64le-linux.