unix: add mips and mipsle to endian files

golang.org/x/sys/unix currently does not compile on mips nor mipsle as it fails to set the `isBigEndian` variable. This is due to mips and mipsle being missing in the build tag of their respective endian files.

This change adds mips to the build tag of `endian_big.go` and mipsle to the build tag of `endian_little.go` to fix this issue.

Change-Id: I79947cedb2645e80143e65bde00f2c95261125fb
Reviewed-on: https://go-review.googlesource.com/39730
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/endian_big.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 //
-// +build ppc64 s390x mips64
+// +build ppc64 s390x mips mips64
 
 package unix
 

unix/endian_little.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 //
-// +build 386 amd64 amd64p32 arm arm64 ppc64le mips64le
+// +build 386 amd64 amd64p32 arm arm64 ppc64le mipsle mips64le
 
 package unix