[gdb/testsuite] Modernize gdb.arch/i386-biarch-core.exp

[gdb/testsuite] Modernize gdb.arch/i386-biarch-core.exp

I noticed in test-case gdb.arch/i386-biarch-core.exp that we run into the
completion limit for "complete set gnutarget":
...
set gnutarget vms-libtxt^M
set gnutarget  *** List may be truncated, max-completions reached. ***^M
(gdb) PASS: gdb.arch/i386-biarch-core.exp: complete set gnutarget
...

Fix this by using get_set_option_choices.

Also use get_set_option_choices for "complete set architecture i386", which
required extending get_set_option_choices to accept a second argument, such
that we can do:
...
set archs [get_set_option_choices "set architecture" "i386"]
...
because this returns an empty list:
...
set archs [get_set_option_choices "set architecture i386"]
...
because it does "complete set architecture i386 ".

Also clean up the explicit gdb_exit/gdb_start and use clean_restart instead.

Tested on x86_64-linux.