[gdb/testsuite] Run gdb.arch/ppc64-symtab-cordic.exp for --enable-targets=all

[gdb/testsuite] Run gdb.arch/ppc64-symtab-cordic.exp for --enable-targets=all

While looking at test-case gdb.arch/ppc64-symtab-cordic.exp I realized that
the test-case is too restrictive here:
...
if {![istarget "powerpc*"] || ![is_lp64_target]} {
    verbose "Skipping powerpc64 separate debug file symtab test."
    return
}
...
and can also be run on x86_64-linux, if "set arch powerpc:common64" is
supported, which is the case if we've build gdb with --enable-targets=all.

Fix this by instead checking if powerpc:common64 is in the completion list for
"set arch".

This allows us to remove the 'untested "powerpc:common64 is not supported"'.

While we're at it, clean up the test-case by using clean_restart.

Tested on x86_64-linux.