Link EFI/uboot loaders with -znotext

By default LLD links with relocations disallowed against readonly
sections (e.g., .text), but the 32-bit ARM EFI & uboot boot bits require
such relocations. -znotext is either ignored as an unknown -z option
(in-tree lld 2.17.50) or is already the default (GNU ld or GNU gold from
ports) so we can just add it unconditionally to allow building with LLD.

This is similar to the change in r320179 for the kernel link.

Sponsored by: The FreeBSD Foundation