cmd/llgo-dist: Add -pepper flag to specify NaCl SDK

If you pass -pepper=path/to/nacl_sdk/pepper_NNN, then
llgo-dist will do the rest (figure out the path to
llvm-config and pnacl-clang, and set the triple.)

There are some other changes thrown in here:
    - Added -x flag to llgo-build (print commands)
    - Added -clang flag to llgo-build (path to clang)
    - Added -a flag to llgo-dist to control rebuilding
      of packages and commands.
Numerous changes to support PNaCl

- Link executables with clang++ (not PNaCl-specific).
- Link PNaCl modules with -lppapi
- Add "pnacl" build tag.
- Remove last fp intrinsic from pkg/math (breaks PNaCl)
- Start porting various std packages to Native Client ABI.

I've got the example module building and loading again,
but it promply crashes due to remaining use of stored
function pointers. I'll fix that in a followup changeset.

To build pkg/nacl/ppapi, do this:

    export GO_CFLAGS="-I/path/to/pepper_canary/include/"
    llgo-build github.com/axw/llgo/pkg/nacl/ppapi
Fixes for non-PNaCl targets