efi/libstub: Move efi_relocate_kernel() into its only remaining user

efi/libstub: Move efi_relocate_kernel() into its only remaining user

LoongArch is the only arch that still uses efi_relocate_kernel(), so
before making changes to it that LoongArch needs, turn it into a private
function. Move efi_low_alloc_above() into mem.c while at it, and drop
the relocate.c source file altogether.

Tested-by: WANG Rui <wangrui@loongson.cn>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>