Adjust Thearch.FREG_MIN/MAX when using non sse2 mode in 8g.

Also, gc.Use_sse is treated as a bool, so make it a bool.

Change-Id: I840411605344bb31c32f492b3e6729166c084f0c
Reviewed-on: https://go-review.googlesource.com/7993Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>