The main change is that #include "zasm_GOOS_GOARCH.h"
is now #include "go_asm.h" and/or #include "go_tls.h".

Also, because C StackGuard is now Go _StackGuard,
the assembly name changes from const_StackGuard to
const__StackGuard.

In asm_$GOARCH.s, add new function getg, formerly
implemented in C.

The renamed atomics now have Go wrappers, to get
escape analysis annotations right. Those wrappers
are in CL 174860043.

LGTM=r, aram
R=r, aram
CC=austin, dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/168510043