[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.

gdb/testsuite/gdb.base/bt-on-fatal-signal.exp

@@ -62,7 +62,10 @@ foreach test_data {{SEGV "Segmentation fault"} \
     with_test_prefix ${sig} {
 
 	# Restart GDB.
-	clean_restart $binfile
+	save_vars { GDB } {
+	    set GDB [gdb_no_core]
+	    clean_restart $binfile
+	}
 
 	# Capture the pid of GDB.
 	set testpid [spawn_id_get_pid $gdb_spawn_id]

gdb/testsuite/lib/gdb.exp

@@ -6322,6 +6322,17 @@ proc cached_file { filename txt {executable 0}} {
     return $filename
 }
 
+# Return a wrapper around gdb that prevents generating a core file.
+
+proc gdb_no_core { } {
+    set script \
+	[list \
+	     "ulimit -c 0" \
+	     [join [list exec $::GDB {"$@"}]]]
+    set script [join $script "\n"]
+    return [cached_file gdb-no-core.sh $script 1]
+}
+
 # Set 'testfile', 'srcfile', and 'binfile'.
 #
 # ARGS is a list of source file specifications.