runtime/wasm: add memset()

Copied from the ARM runtime and modified to return a pointer.
https://pubs.opengroup.org/onlinepubs/9699919799/functions/memset.html

src/runtime/runtime_wasm.go

@@ -2,6 +2,10 @@
 
 package runtime
 
+import (
+	"unsafe"
+)
+
 type timeUnit int64
 
 const tickMicros = 1
@@ -48,3 +52,11 @@ func ticks() timeUnit {
 func abort() {
 	trap()
 }
+
+//go:export memset
+func memset(ptr unsafe.Pointer, c byte, size uintptr) unsafe.Pointer {
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(unsafe.Pointer(uintptr(ptr) + i)) = c
+	}
+	return ptr
+}