It was using lstat to get the size of /proc/self/exe but
it always returns 0, so we just use a big buffer on the
stack instead.

BUG=angleproject:892

Change-Id: I6d88efeb4ec5de7a78cb3668e3d78520203ad1d5
Reviewed-on: https://chromium-review.googlesource.com/269990Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>