EFI: Initial test EFI application

* Only set HAIKU_BOOT_PLATFORM to bios_ia32 if not defined
* Add gnuefi build feature
* Introduce BOOT_LDFLAGS, and move options for passing to linker
into ArchitectureSetup
* x86_64 compile fixes for warnings in boot loader
* loader/elf.cpp: don't include ELF32 support when targeting EFI
* relocation_func.cpp: copy of the relocation code from gnuefi
to make _relocate extern "C", and avoid including <efilib.h>
* boot_loader_efi.ld: copy of gnuefi's elf_x86_64_efi.lds,
modified to include support for C++ constructors, etc. Keep in
sync with the gnuefi package

Signed-off-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>