efi/libstub: Re-enable command line initrd loading for x86

Commit:

  cf6b8366 ("efi/libstub: Make initrd file loader configurable")

inadvertently disabled support on x86 for loading an initrd passed via
the initrd= option on the kernel command line.

Add X86 to the newly introduced Kconfig option's title and depends
declarations, so it gets enabled by default, as before.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-efi@vger.kernel.org