cmd/dist: Plan 9 build needs an additional include path

cmd/cc: bv.c imports libc.h twice

When using the Plan 9 compiler, the invocation

        #include <../ld/textflag.h>

works for the toolchain, but not for the MACH library.

Module cmd/cc/bv.c includes libc.h and "cc.h", which in
turn also includes libc.h.  In the Plan 9 context, this
causes a number of duplicate definitions.

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/13303047

src/cmd/cc/bv.c

@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 #include <u.h>
-#include <libc.h>
 #include "cc.h"
 
 enum {

src/cmd/dist/build.c

@@ -932,6 +932,8 @@ install(char *dir)
 					vadd(&compile, "-Bp+");
 				vadd(&compile, bpathf(&b, "-I%s/include/plan9", goroot));
 				vadd(&compile, bpathf(&b, "-I%s/include/plan9/%s", goroot, gohostarch));
+				// Work around Plan 9 C compiler's handling of #include with .. path.
+				vadd(&compile, bpathf(&b, "-I%s/src/cmd/ld", goroot));
 			} else {
 				vcopy(&compile, gccargs.p, gccargs.len);
 				vadd(&compile, "-c");