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

[dim]

Summary

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.

To avoid duplicating the .SUFFIXES list and suffix-transformation rules
from bsd.lib.mk, move these to bsd.suffixes-extra.mk so they can be
included from both bsd.lib.mk and bsd.prog.mk. (Note that we cannot add
them to bsd.suffixes.mk, since that file also gets included by sys.mk,
which applies to non-source builds. That would require a whole ports
exp-run.)

MFC after: 1 week

[dim]

Note this requires D49951 to be committed first, since otherwise tests/sys/fs/fusefs falls over due to it picking sbin/mount.c over tests/sys/fs/fusefs/mount..cc for targeting mount.pieo. This has to do with some internal ordering in make that I haven't been able to fully figure out.

[emaste]

Are you up for splitting bsd.suffixes-extra.mk out first, without any functional changes?

[dim]

In D49962#1139420, @emaste wrote:

Are you up for splitting bsd.suffixes-extra.mk out first, without any functional changes?

Sure, that should be a very safe NFC operation. :)

[dim]

Split off the bsd.suffixes-extra.mk part to D49965.

[dim]

Abandoning, arc diff totally messed this up.