Fix cross-builds from macOS

The macOS linker does not support -zrelro/-znorelro. Since it is only
used to for build tools that run on the host, and WITH_RELRO or
WITHOUT_RELRO does not matter there, just skip the option.

Reviewed by: markj
Fixes: 2f3a961487c9 ("Add RELRO build knob, default to enabled")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35589

(cherry picked from commit 7927452ac4e98309bd832d55526cc98438fba8a5)