CL 122515 tightened TestAbort to look for breakpoint exceptions and
not just general signal crashes, but this only applies on x86 arches.
On non-x86 arches we use a nil pointer dereference to abort, so the
test is now failing.
This CL re-loosens TestAbort on non-x86 arches to only expect a signal
traceback.
Should fix the build on linux/arm, linux/arm64, linux/ppc64, and
linux/s390x.