devel/efivar: exorcise -march=native

This fixes the build on powerpc64le.
Should also avoid the usual problems caused by -march=native.

Approved by: portmgr (build fix blanket)
MFH: 2026Q1
Sponsored by: Raptor Computing Systems, LLC

(cherry picked from commit 31cb697698ac528a3836564ffbaf68604866f622)