unix: emit +build comments in generator scripts

Change-Id: I8ed64dcb1c1a2119ce6f15fa1be55fbe0b0c637e
Reviewed-on: https://go-review.googlesource.com/10087
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

unix/mkerrors.sh

@@ -11,6 +11,11 @@ unset LANG
 export LC_ALL=C
 export LC_CTYPE=C
 
+if test -z "$GOARCH" -o -z "$GOOS"; then
+  echo 1>&2 "GOARCH or GOOS not defined in environment"
+  exit 1
+fi
+
 CC=${CC:-gcc}
 
 uname=$(uname)
@@ -347,6 +352,8 @@ echo '#include <signal.h>' | $CC -x c - -E -dM $ccflags |
 echo '// mkerrors.sh' "$@"
 echo '// MACHINE GENERATED BY THE COMMAND ABOVE; DO NOT EDIT'
 echo
+echo "// +build ${GOARCH},${GOOS}"
+echo
 go tool cgo -godefs -- "$@" _const.go >_error.out
 cat _error.out | grep -vf _error.grep | grep -vf _signal.grep
 echo