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.

Bug: 28740702

(cherry picked from commit 533dde4d)

Change-Id: Ied16987756a501acf292368a14e3727ad631efa5