Don't install SIGILL handler until after sigsetjmp.

Previously, if SIGILL was received between the signal handler being
installed and before the sigsetjmp, the process would longjmp to a
random location.

Change-Id: I9e6143a17ff3db0e1b00ece68fce161801461010
Reviewed-on: https://boringssl-review.googlesource.com/3950
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>