loader: libefi/env.c warnings in arm build

The arm build has revealed some of the warnings, the fix for CHAR16
warning is to switch the warning off for env.c (same as for efinet.c).

For error code we need to use macro.

Reported by: gjb
Reviewed by: imp
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D9422