arm64: efi: Move efi-entry.S into the libstub source directory

We will be sharing efi-entry.S with the zboot decompressor build, which
does not link against vmlinux directly. So move it into the libstub
source directory so we can include in the libstub static library.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>