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

ui/build/Android.bp

@@ -66,11 +66,13 @@ bootstrap_go_package {
     ],
     darwin: {
         srcs: [
+            "config_darwin.go",
             "sandbox_darwin.go",
         ],
     },
     linux: {
         srcs: [
+            "config_linux.go",
             "sandbox_linux.go",
         ],
     },