Previously we were writing large numbers of zeros to the output
file. This change only writes out the initialized portion and
allocates the full address space at runtime. This reduces compile time
by around 50%.

This change also adds a couple of WASM-specific timers.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4369
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1938643002 .