Only use tmpfs if build machine has 4GB+ RAM.

For machines with low RAM (or no access to /proc/meminfo) the builder
will still run, but will just build inside the filesystem that is
hosting /tmp. This will result in a slower build (especially if there
are a lot of .deb packages installed).

Fixes bug #1175453

Change-Id: I79f2672058c11e377548820df0ab4fad8f47ffdc