Better handling of low memory situations

Drop down to a single high-mem task when we've got <=16GB, as the system
probably isn't completely free RAM, ninja will be using a couple gigs,
along with whatever "normal" actions will be running concurrently. So
it's unlikely that we can handle two 6-8GB actions along with everything
else.

Also print warnings when we detect <=16GB total RAM, or when we're
running more parallel jobs than than we have GB RAM. These both notify
the user and suggest lowering the `-j` value if they run into problems.

Test: fake totalRAM to [0.5,8,16]GB, checking warning
Test: fake totalRAM to 17GB, `m -j4 nothing` has no warning
Test: `m -j187 nothing` on a 188GB system
Test: `m -j188 nothing` on a 188GB system
Change-Id: Ieb008e9f462d5f40fb65781d94cf116b1caf8446