wasm: remove i64 workaround, use BigInt instead

Browsers previously didn't support the WebAssembly i64 type, so we had
to work around that limitation by converting the LLVM i64 type to
something else. Some people used a pair of i32 values, but we used a
pointer to a stack allocated i64.

Now however, all major browsers and Node.js do support WebAssembly
BigInt integration so that i64 values can be passed back and forth
between WebAssembly and JavaScript easily. Therefore, I think the time
has come to drop support for this workaround.

For more information: https://v8.dev/features/wasm-bigint (note that
TinyGo has used a slightly different way of passing i64 values between
JS and Wasm).

For information on browser support: https://webassembly.org/roadmap/

builder/build.go

@@ -1054,17 +1054,6 @@ func optimizeProgram(mod llvm.Module, config *compileopts.Config) error {
 		return err
 	}
 
-	// Browsers cannot handle external functions that have type i64 because it
-	// cannot be represented exactly in JavaScript (JS only has doubles). To
-	// keep functions interoperable, pass int64 types as pointers to
-	// stack-allocated values.
-	if config.WasmAbi() == "js" {
-		err := transform.ExternalInt64AsPtr(mod, config)
-		if err != nil {
-			return err
-		}
-	}
-
 	// Optimization levels here are roughly the same as Clang, but probably not
 	// exactly.
 	optLevel, sizeLevel, inlinerThreshold := config.OptLevels()