runtime: fix nacl/amd64p32 build

Update issue 8675
Fixes nacl/amd64p32 build.

TBR=khr
CC=golang-codereviews
https://codereview.appspot.com/141140045
cmd/dist: remove goc2c

After the three pending CLs listed below, there will be no more .goc files.

134580043	runtime: move stubs.goc code into runtime.c
133670043	runtime: fix windows syscalls for copying stacks
141180043	runtime: eliminate Go -> C -> block paths for Solaris

LGTM=bradfitz
R=golang-codereviews, bradfitz, dave
CC=golang-codereviews, iant, r
https://codereview.appspot.com/132680043