cdata: cleanup and go 1.3 / 1.4 fixes

Moved all the assembly into cdata_{386,arm,amd64}.s files:
 - still 3 files
 - more familiar arrangement
 - simpler build constraints
 - no need for amd64p32 anyway

The go1.4 version of Addrs is now just written in assembly using multiple
return values.

Now that tip has a go1.4 tag, I can re-introduce the go 1.3 code with
a !go1.4 build constraint so that both 1.3 and 1.4 should now work
properly. The go1.3 code is in cdata_go13.{c,go}.