Update sigcontext and siginfo structs to match those currently in use by OpenBSD armv7.

Also correct the offset of the fault address field in the siginfo struct, which moved
due to the switch to EABI.

Change-Id: Icdd95222346239fcc04b95ae0fcefae09b7aa044
Reviewed-on: https://go-review.googlesource.com/c/154077Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>