cmd/dist: drop unneeded clang flags

Our source no longer needs these flags set to build cleanly using clang.

Tested with

* Ubuntu clang version 3.0-6ubuntu3 (tags/RELEASE_30/final) (based on LLVM 3.0) on i386
* clang version 3.2 (tags/RELEASE_32/final) on amd64 cross compiling all platforms

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7058053