Call `set disable-randomization off` only if it seems to be supported.
The goal is to neither get an error about disable-randomization not
being supported (e.g. on FreeBSD), nor get an error if it is supported
but fails (e.g. on Ubuntu).

Only fiddle with disable-randomization from all.sh, which cares
because it reports the failure of ASLR disabling as an error. If a
developer invokes the Gdb script manually, a warning about ASLR
doesn't matter.