This reduces complexity in the compiler without affecting binary sizes
too much.
Cortex-M0: no changes
Linux x64: no changes
WebAssembly: some testcases (calls, coroutines, map) are slightly bigger
This simplifies the ABI a lot and makes future changes easier.
In the future, determining which functions need a context parameter
should be moved from IR generation into an optimization pass, avoiding
the need for recursively scanning the Go SSA.
To support the WebAssembly<->JS barrier, return values also have to be
passed in memory. i64 return values are used by syscall/js, so must be
supported across this ABI barrier.
JavaScript does not support i64 directly, so make sure we pass a pointer
instead which can be read from JavaScript.
This is a temporary workaround which should be removed once JavaScript
supports some form of i64 (probably in the form of BigInt).
