[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.