asm parser lacking `label` parse functionality

[badumbatish]

In rust's asm.rs I misunderstood eat_keyword(label) for "tracks if there is any number that is followed by a :" to be a label. The former is explicitly a label and the latter is a local label.

The former needs to be parsed explicitly and put in a hashmap or set to be expanded later.

The latter can be implicitly parsed via parse_format_strings, it is passed straight into the assembler.

For docs rust zulip asm discussion arm's docs on local label rust's asm goto rust's tracking issue