unix: define isBigEndian for all GOARCHes supported by gccgo

For golang/go#18031
For golang/go#37443

Change-Id: I49dabb362592bb61532a0c4f193919c3f2036af3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/270317
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@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 mips mips64
+// +build armbe arm64be m68k mips mips64 mips64p32 ppc ppc64 s390 s390x shbe sparc sparc64
 
 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 mipsle mips64le riscv64
+// +build 386 amd64 amd64p32 alpha arm arm64 mipsle mips64le mips64p32le nios2 ppc64le riscv riscv64 sh
 
 package unix