gc_leaking: Don't zero out new allocations in some targets (#4302)

Wasm linear memory is always initialized to zero by definition,
so there's no need to waste time zeroing out this allocation. This
is the case for freshly-obtained memory from mmap().

Signed-off-by: L. Pereira <l.pereira@fastly.com>

src/runtime/gc_leaking.go

@@ -44,7 +44,7 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 		runtimePanic("out of memory")
 	}
 	pointer := unsafe.Pointer(addr)
-	memzero(pointer, size)
+	zero_new_alloc(pointer, size)
 	return pointer
 }