Add TotalRAM detection

Colin originally wrote this for the highmem pool:

https://android-review.googlesource.com/c/platform/build/soong/+/1168271

But while that's a larger change, this is useful to just be in the logs,
and for use in multiproduct_kati to better limit the number of
concurrent jobs.

Test: check soong.log on linux and mac
Change-Id: I0518d303a220d775f8d78dba9f660b2954e68e3e