bsd.lib.mk: add conditions for building _pie.a archives

bsd.lib.mk: add conditions for building _pie.a archives

As with other .a targets, build _pie.a archives only if LIB is set.

At present we build _pie.a only for INTERNALLIBs, and none of them
include bsd.lib.mk without setting LIB.  However, we might want to build
_pie.a for non-INTERNALLIBs in the future.

Reviewed by:	arichardson
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D31920

(cherry picked from commit 7c0226cad3f36a05832f9c5216dfa3dadb91c92d)