* 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
The result of the emitter is a vector of bytes holding machine code,
jump tables, and (in the future) other read-only data. Some clients,
notably Firefox's Wasm compiler, needs to separate the machine code
from the data in order to insert more code directly after the code
generated by Cranelift.
To make such separation possible, we record more information about the
emitted bytes: the sizes of each of the sections of code, jump tables,
and read-only data, as well as the locations within the code that
reference (PC-relatively) the jump tables and read-only data.
