Use $(pie-default) with conformtest

My glibc bot showed that my conformtest changes fail the build of the
conformtest execution tests for x86_64-linux-gnu-static-pie, because
linking the newly built object with the newly built libc and the
associated options normally used for linking requires it to be built
as PIE.  Add $(pie-default) to the compiler command used so that PIE
options are used when required.

There's a case for using the whole of $(CFLAGS-.o) (which includes
$(pie-default)), but that raises questions of any impact from using
optimization flags from CFLAGS in these tests.  So for now just use
$(pie-default) as the key part of $(CFLAGS-.o) that's definitely
needed.

Tested with build-many-glibcs.py for x86_64-linux-gnu-static-pie.

(cherry picked from commit 885762aa)