Explicitly check for linking format to use for efiemu64 module

Explicitly check for linking format to use for efiemu64 module

Similar to check for target linking format, also check for efiemu64
instead of hardcoding -melf_x86_64. This fixes compilation on *BSD
variants. We cannot easily reuse main target check because platforms
are different (main target is 32 bit and efiemu64 - 64 bit).

This commit adds EFIEMU64_LINK_FORMAT that contains detected
link option and is used in efiemu64.o linking instead of hardcoded
value.

Reported-By: Beeblebrox <zaphod@berentweb.com>