runtime: fix misaligned 64-bit atomic
Fixes issue 4869.
Fixes issue 5007.
Update issue 5005.

R=golang-dev, 0xe2.0x9a.0x9b, bradfitz
CC=golang-dev
https://codereview.appspot.com/7534044
cmd/ld: add tmpdir flag to preserve temp files

R=ken2
CC=golang-dev
https://codereview.appspot.com/7497044