go.sys/all: update #includes to new location

Darwin/amd64 now builds. No others tested yet.
mkall.sh also runs correctly. The updates, which are
substantial, are not yet applied to avoid unnecessary churn.
Restore mkerrors.sh, which is needed in the unix directory.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/122410044

plan9/asm_plan9_386.s

@@ -5,7 +5,7 @@
 // TODO(rsc): Rewrite all nn(SP) references into name+(nn-8)(FP)
 // so that go vet can check that they are correct.
 
-#include "../../cmd/ld/textflag.h" // TODO: How to refer to this?
+#include "textflag.h"
 
 //
 // System call support for 386, Plan 9

plan9/asm_plan9_amd64.s

@@ -5,7 +5,7 @@
 // TODO(rsc): Rewrite all nn(SP) references into name+(nn-8)(FP)
 // so that go vet can check that they are correct.
 
-#include "../../cmd/ld/textflag.h" // TODO: How to refer to this?
+#include "textflag.h"
 
 //
 // System call support for Plan 9