[gdb/testsuite] Use pie/nopie instead of ldflags=-pie/-no-pie

I noticed two test-case that use ldflags=-pie and ldflags-no-pie, instead of
the canonical pie and nopie options, which would typically also add
additional_flags=-fPIE respectively additional_flags=-fno-pie.

There is no indication that this is on purpose, so replace these with pie and
nopie.

Tested on x86_64-linux.

gdb/testsuite/gdb.base/load-command.exp

@@ -20,7 +20,7 @@ standard_testfile
 # Disable generation of position independent executable (PIE).  Otherwise, we
 # would have to manually specify an offset to load.
 
-set opts {debug ldflags=-no-pie}
+set opts {debug nopie}
 
 if [prepare_for_testing "failed to prepare" $testfile $srcfile $opts] {
     return -1

gdb/testsuite/gdb.dwarf2/fission-loclists-pie.exp

@@ -40,7 +40,7 @@ standard_testfile .S
 set obj [standard_output_file "${testfile}.o"]
 set dwo [standard_output_file "${testfile}.dwo"]
 if [build_executable_and_dwo_files "$testfile.exp" "${binfile}" \
-	{nodebug ldflags=-pie} \
+	{nodebug pie} \
 	[list $srcfile [list nodebug split-dwo additional_flags=-DDWO=\"$dwo\"] \
 	     $obj]] {
     return -1