runtime: translate env*.c to Go

In an earlier CL I wrote a separate Go-only version, but that broke Plan 9,
because the Go-only version assumed a non-Plan 9 system.

Translate the real ones instead.

LGTM=r
R=golang-codereviews, r
CC=0intro, golang-codereviews, iant, khr
https://codereview.appspot.com/140050044
cmd/cc: generate error if #pragma pack off does anything

We can't translate misaligned things to Go, so start rejecting them in C.

The only one in any build appears to be EpollEvent on linux/amd64.
Fix that.

LGTM=r
R=golang-codereviews, r, dvyukov
CC=golang-codereviews, iant
https://codereview.appspot.com/137020043