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

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