[gdb/testsuite] Enable gdb.arch/s390-disassembler-options.exp for --enable-targets=all

[gdb/testsuite] Enable gdb.arch/s390-disassembler-options.exp for --enable-targets=all

On s390x-linux, I run into:
...
DUPLICATE: gdb.arch/s390-disassembler-options.exp: \
  show disassembler-options esa
...

First, reproduce this on x86_64-linux with --enable-targets=all, by replacing
the test for 'istarget "s390*-*-*"' with a test for 'get_set_option_choices
"set architecture" "s390"'.

Fix the DUPLICATE by using with_test_prefix.

Also modernize the test-case by using clean_restart instead of gdb_exit/gdb_start.

Tested on x86_64-linux.