Fixes #10450

runtime.cputicks is called from runtime.exitsyscall and must not
split the stack. cputicks is implemented in several ways and the
NOSPLIT annotation was missing from a few of these.

Change-Id: I5cbbb4e5888c5d298fe2fef240782d0e49f59af8
Reviewed-on: https://go-review.googlesource.com/8939Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>