efi_loader: Rename EFI_RUNTIME_{TEXT, DATA} to __efi_runtime{, _data}

Compiler attributes are more commonly __foo style tags rather than big
upper case eye sores like EFI_RUNTIME_TEXT.

Simon Glass felt quite strongly about this, so this patch converts our
existing defines over to more eye friendly ones.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>