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/config_linux.go

@@ -0,0 +1,28 @@
+// Copyright 2019 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package build
+
+import "syscall"
+
+func detectTotalRAM(ctx Context) uint64 {
+	var info syscall.Sysinfo_t
+	err := syscall.Sysinfo(&info)
+	if err != nil {
+		ctx.Printf("Failed to get system memory size: %s")
+		return 0
+	}
+	memBytes := uint64(info.Totalram) * uint64(info.Unit)
+	return memBytes
+}