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