cmd/go: update PE header magic for files generated by GNU ld

Change-Id: I2d2ea233f976aab3f356f9b508cdd246d5013e32
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7536Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -1267,7 +1267,7 @@ var objectMagic = [][]byte{
 	{0xFE, 0xED, 0xFA, 0xCF},                  // Mach-O big-endian 64-bit
 	{0xCE, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 32-bit
 	{0xCF, 0xFA, 0xED, 0xFE},                  // Mach-O little-endian 64-bit
-	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00, 0x04, 0x00}, // PE (Windows) as generated by 6l/8l
+	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00},      // PE (Windows) as generated by 6l/8l and gcc
 	{0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
 	{0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
 }