gdb/testsuite: expect SIGSEGV from top GDB spawn id

gdb/testsuite: expect SIGSEGV from top GDB spawn id

When testing with the native-extended-gdbserver, I get:

    Thread 1 "xgdb" received signal SIGSEGV, Segmentation fault.
    0x00007ffff6d828f2 in GC_find_limit_with_bound () from /usr/lib/x86_64-linux-gnu/libgc.so.1
    (gdb) FAIL: gdb.gdb/selftest.exp: xgdb is at prompt

This is because the -re that is supposed to match this SIGSEGV is after
`-i $inferior_spawn_id`.  On native, the top and bottom GDB are on the
same spawn id, so it ends up working.  But with a gdbserver board,
that's not the case.  Move the SIGSEGV -re before the `-i
$inferior_spawn_id` line, such that it matches what the top GDB outputs.

Do the same fix in gdb.gdb/python-helper.exp.

Change-Id: I3291630e218a5a3a6a47805b999ddbc9b968c927
Approved-By: Tom Tromey <tom@tromey.com>