efi_loader: correctly set the machine type in the PE header

The portable executable header has a field describing the machine type.
The machine type should match the binary. So on i386 we should use
IMAGE_FILE_MACHINE_I386 and on x86_64 we should use
IMAGE_FILE_MACHINE_AMD64. The actual value is issued by the objcopy
command invoked in scripts/Makefile.lib in depdendence of the value of
EFI_TARGET.

The value is used both for EFI_STUB and for EFI_LOADER.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>