Fix a test broken by r216564.

llgs Linux is no longer doing the translation to some gdb fixed signal
numbers.  This change modifies the test method to take in the signo
expected for a segfault.  The debugserver test uses the fixed gdb number,
and everything else uses signal.SIGSEGV for the platform.

Sidenote: I somehow did not see this reported in multicore tests.
I will need to verify that we aren't missing test failures in that
script.  I need to verify why this wasn't more obvious with
test/dosep.py.