wasm32-freestanding imports memset instead of defining it

[fengb]

When targeting wasm32-freestanding, Zig uses memset for certain operations, like assigning undefined. Instead of pulling in a statically linked version, Zig now requires an imported function instead:

export fn blah() void {
 var foo: [1000]u8 = undefined;
}

(module
 (type (;0;) (func (param i32 i32 i32) (result i32)))
 (type (;1;) (func))
 (import "env" "memset" (func $memset (type 0)))
 (func $__wasm_call_ctors (type 1))
 (func $blah (type 1)

Looks like this regressed due to #6250

[andrewrk]

@alexnask be careful tagging stuff with the wrong stage1/stage2 thing pls, I almost didn't notice this regression