This removes the explicit dependency on target-lexicon for the embedder,
which can instead use the ISA's name directly. It can simplify
dependency management, in particular avoid the need for synchronizing
the target-lexicon dependencies versions.
It also tweak the error when an ISA isn't built as part of Cranelift to
be a SupportDisabled error; this was dead code before this.
* the target-lexicon crate no longer has or needs the std feature
in cargo, so we can delete all default-features=false, any mentions
of its std feature, and the nostd configs in many lib.rs files
* the representation of arm architectures has changed, so some case
statements needed refactoring
