Use __ as the separator for the exported vars in bsd.compiler/linker.mk

By using '__' instead of '.' as the separator we can also support systems
that use dash as /bin/sh (it's the default shell on Ubuntu/Debian). Dash
will unset any environment variables that use a non alphanumeric+undedscore
character and therefore submakes will fail to import the COMPILER_*
variables if we use '.' as the separator.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D22381