Support CLOCK_BOOTTIME in QElapsedTimer.

CLOCK_MONOTONIC does not include time spent in suspend, which is particularly
problematic on e.g. Android systems, which suspend often. CLOCK_BOOTTIME was
introduced in Linux 2.6.39 addressing exactly this problem: it is identical to
CLOCK_MONOTONIC, but also includes time spent in suspend.

Change-Id: I679e788cbe3ae2a2d5be59ed99427678f09cc032