share/mk: Define a rule for dtrace -G to generate .pieo files

Now that bsd.prog.mk uses .pieo instead of .o for object files linked
into PIEs, we need to ensure that the object file generated by dtrace -G
has the same extension.

Fixes: 245bb0110639 ("Use .pieo extension for WITH_PIE bsd.prog.mk output")
Sponsored by: Innovate UK