runtime: correct misplaced right brace in Linux SIGBUS handling

I'm not sure how to write a test for this.  The change in
behaviour is that if you somehow get a SIGBUS signal for an
address >= 0x1000, the program will now crash rather than
calling panic.  As far as I know, on x86 GNU/Linux, the only
way to get a SIGBUS (rather than a SIGSEGV) is to set the
stack pointer to an invalid value.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7906045