ninja generator: run even if "/proc/meminfo" is missing

Linux may *not* have "/proc/meminfo" available, and especially
not in a chroot (GBS case). Make it fall back gracefully.

Origin: https://codereview.chromium.org/137623016/patch/1/10001

Change-Id: Iabb78e79fe1dd4c1a936530f6b8d35c0b7ed2ecf
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>