[dev.cc] cmd/dist: adjust for build process without cmd/cc

[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

- Remove references to C compiler directories.
- Remove generation of special header files.
- Remove generation of Go source files from C declarations.

- Compile Go sources before rest of package (was after),
  so that Go compiler can write go_asm.h for use in assembly.

- Move TLS information from cmd/dist (was embedding in output)
  to src/runtime/go_tls.h, which it can be maintained directly.

LGTM=r
R=r, dave
CC=austin, golang-codereviews, iant, khr
https://golang.org/cl/172960043