Use .pieo extension for WITH_PIE bsd.prog.mk output

When object files for programs are built using bsd.prog.mk, and WITH_PIE
is enabled, the extension used is still plain ".o". To be consistent
with bsd.lib.mk, and to allow changes in WITH_PIE settings to propagate
correctly, the extension should be ".pieo" instead.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49966

(cherry picked from commit 245bb0110639fe963086fb2903143dbd4e6ac48c)