csu: define STRIP_FBSDID

__FBSDID() places the provided string in the output object's .comment
section. However, with the transition to Git $FreeBSD$ is no longer
expanded and so we emitted a literal $FreeBSD$.

$FreeBSD$ will be addressed in a holistic manner in the future, but at
least avoid embedding it into everything linked on FreeBSD (via csu).

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33594