gdb.base/exitsignal.exp: Exit with non-zero

gdb.base/exitsignal.exp currently tests exiting with exit code 0.
But, testing for 0 is typically more "dangerous" in the sense that
it's easy for some bug in GDB or the target backend to fail to extract
the exit code and return 0.  Make it test exit code 1 instead.

gdb.base/coredump-filter-build-id.exp currently reuses normal.c.  It
could still use it with this patch, but it seems better to me to
remove the coupling.  Simply add a new .c file for that testcase,
which still returns 0.

Change-Id: I49ccfdbbe4be4445172476e7dd4e06142c8a672e