[gdb/testsuite] Require can_spawn_for_attach in gdb.base/gstack.exp

I ran test-case gdb.base/gstack.exp on a machine with kernel.yama.ptrace_scope
set to 1 and ran into:
...
PASS: gdb.base/gstack.exp: spawn gstack
ptrace: Operation not permitted.^M
GSTACK-END^M
PASS: gdb.base/gstack.exp: gstack exits with no error
PASS: gdb.base/gstack.exp: gstack's exit status is 0
FAIL: gdb.base/gstack.exp: got backtrace
...

Fix this by requiring can_spawn_for_attach.

Tested on x86_64-linux.