cmd/go/internal: add wasm magic header

This is needed when we are re-building to the wasm target.

Fixes #25774

Change-Id: I5dc47bd7e56c8570b8d2485273ef0d10f2feac6f
Reviewed-on: https://go-review.googlesource.com/116915Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/work/exec.go

@@ -1565,6 +1565,7 @@ var objectMagic = [][]byte{
 	{0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
 	{0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
 	{0x00, 0x00, 0x06, 0x47},                  // Plan 9 arm
+	{0x00, 0x61, 0x73, 0x6D},                  // WASM
 }
 
 func isObject(s string) bool {