efi_loader: use freestanding library for efi apps

GCC requires that freestanding programs provide memcpy(), memmove(),
memset(), and memcmp().

Add the library functions when building a *.efi files.

The EFI selftests might use other compilation flags. So use a symbolic
link to provide lib/efi_selftest/efi_freestanding.c and compile it
separately.

Reported-by: Alexander Graf <agraf@suse.de>
Fixes: 5be444d14b38 ("efi_loader: consistent build flags for EFI applications")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>