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.