bsd.lib.mk: decouple lib*_pic.a from TOOLCHAIN build knob

A user may use a tool chain from a package or just use an existing
tool chain from a previous installation. There is no reason for this
to disable the installation of lib${LIB}_pic.a.

This also means we don't need to force MK_TOOLCHAIN=yes in lib/libc.

This reverts part of commit c0f5aeb0329d71e6b02379133c0c9c0145c9afea.

Reviewed by: jrtc27
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39917