Fix brk/sbrk error checking.

Note that the kernel returns the current break on error or if the requested
break is smaller than the minimum break, or the new break. I don't know where
we got the idea that the kernel could return -1.

Also optimizes the query case.

Also hides an accidentally-exported symbol for LP64.

Change-Id: I0fd6b8b14ddf1ae82935c0c3fc610da5cc74932e