Relax ordering rules for `asm!` operands
The `asm!` and `global_asm!` macros require their operands to appear strictly in the following order:
- Template strings
- Positional operands
- Named operands
- Explicit register operands
- `clobber_abi`
- `options`
This is overly strict and can be inconvienent when building complex `asm!` statements with macros. This PR relaxes the ordering requirements as follows:
- Template strings must still come before all other operands.
- Positional operands must still come before named and explicit register operands.
- Named and explicit register operands can be freely mixed.
- `options` and `clobber_abi` can appear in any position after the template strings.
r? ```````@joshtriplett```````
