[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.

gdb/testsuite/gdb.arch/ppc64-symtab-cordic.exp

@@ -13,7 +13,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-if {![istarget "powerpc*"] || ![is_lp64_target]} {
+clean_restart
+
+set supported_archs [get_set_option_choices "set architecture"]
+if { [lsearch -exact $supported_archs "powerpc:common64"] == -1 } {
     verbose "Skipping powerpc64 separate debug file symtab test."
     return
 }
@@ -34,10 +37,6 @@ if {[catch "system \"bzip2 -dc ${kodebugbz2file} >${kodebugfile}\""] != 0} {
     return -1
 }
 
-gdb_exit
-gdb_start
-gdb_reinitialize_dir $srcdir/$subdir
-
 # This test won't work properly if system debuginfo is installed.
 # Test message is suppressed by "" as otherwise we could print PASS+UNTESTED
 # result to gdb.sum making a false feeling the issue has been tested.
@@ -50,7 +49,4 @@ gdb_test_multiple $test $test {
     -re "\r\nThe target architecture is set to \"auto\" \\(currently \"powerpc:common64\"\\)\.\r\n$gdb_prompt $" {
 	pass $test
     }
-    -re "\r\nThe target architecture is set to \"auto\" \\(currently \".*\"\\)\.\r\n$gdb_prompt $" {
-	untested "powerpc:common64 is not supported"
-    }
 }