spl: Unbreak crossbuild after f20893853e8e

Summary:
f20893853e8e made getexecname no longer guarded by BOOTSTRAPPING, but it
does not build on non-FreeBSD as it tries to use KERN_PROC_PATHNAME and
related constants.

Reviewers: mm, imp

Reviewed By: imp

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D30763