Update Go so that it continues to work past the OpenBSD system ABI
break, with 64-bit time_t:

  http://www.openbsd.org/faq/current.html#20130813

Note: this makes OpenBSD 5.5 (currently 5.4-current) the minimum
supported release for Go.

Fixes #7049.

R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/13368046