[gdb/testsuite] Don't generate core in gdb.base/bt-on-fatal-signal.exp

[gdb/testsuite] Don't generate core in gdb.base/bt-on-fatal-signal.exp

When running test-case gdb.base/bt-on-fatal-signal.exp on powerpc64le-linux I
noticed:
...
FAIL: gdb.base/bt-on-fatal-signal.exp: SEGV: scan for backtrace (timeout)
...

The timeout is 10 seconds, but generating the core file takes more than a
minute, probably due to slow NFS.

I managed to reproduce this behaviour independently of gdb, by compiling
"int main (void) { __builtin_abort (); }" and running it, which took 1.5
seconds for a core file 50 times smaller than the one for gdb.

Fix this by preventing the core file from being generated, using a wrapper
around gdb that does "ulimit -c 0".

Tested on x86_64-linux.