syscall: ignore godefs input when building on Plan 9

R=golang-dev, akumar, rsc
CC=golang-dev
https://golang.org/cl/5534055

src/buildscript_plan9_386.sh

@@ -263,9 +263,8 @@ cp "$WORK"/encoding/json.a "$GOROOT"/pkg/plan9_386/encoding/json.a
 mkdir -p "$WORK"/syscall/_obj/
 cd "$GOROOT"/src/pkg/syscall
 8g -o "$WORK"/syscall/_obj/_go_.8 -p syscall -I "$WORK" ./env_plan9.go ./exec_plan9.go ./str.go ./syscall.go ./syscall_386.go ./syscall_plan9.go ./syscall_plan9_386.go ./zerrors_plan9_386.go ./zsyscall_plan9_386.go ./zsysnum_plan9_386.go ./ztypes_plan9_386.go
-8c -FVw -I "$WORK"/syscall/_obj/ -I "$GOROOT"/pkg/plan9_386 -o "$WORK"/syscall/_obj/types_plan9.8 -DGOOS_plan9 -DGOARCH_386 ./types_plan9.c
 8a -I "$WORK"/syscall/_obj/ -o "$WORK"/syscall/_obj/asm_plan9_386.8 -DGOOS_plan9 -DGOARCH_386 ./asm_plan9_386.s
-gopack grc "$WORK"/syscall.a "$WORK"/syscall/_obj/_go_.8 "$WORK"/syscall/_obj/types_plan9.8 "$WORK"/syscall/_obj/asm_plan9_386.8
+gopack grc "$WORK"/syscall.a "$WORK"/syscall/_obj/_go_.8 "$WORK"/syscall/_obj/asm_plan9_386.8
 cp "$WORK"/syscall.a "$GOROOT"/pkg/plan9_386/syscall.a
 
 #

src/pkg/syscall/types_plan9.c

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 /*
 Input to godefs.  See also mkerrors.sh and mkall.sh
 */