ld: don't hardcode "-melf_i386"

On FreeBSD, the name of the emulation is "-melf_i386_fbsd", so allow
SeaBIOS to fetch the emulation to use from the environment variables.

This allows SeaBIOS to compile on FreeBSD with gcc.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>