Use canonical __PIC__ flag

It is automatically set when -fPIC is passed to the compiler.

Reviewed by:	dim, kib
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1179


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274772 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f