Add path interposer

This will allow us to track (and eventually limit) the commands that the
build references via $PATH. These are mostly implicit dependencies on
the host system -- for Linux, we assume something similar to Ubuntu
14.04 with a few extra packages, but this will let us better define
that.

This will not catch uses of tools with absolute paths (/bin/bash, etc),
but most uses shouldn't be relying on absolute path names anyways.

Adds ~400ms on the first startup, ~140ms on subsequent runs, and
overhead of a few ms for every forwarded execution.

Test: m
Test: build/soong/build_test.bash
Test: Add `gcc --version`, TEMPORARY_DISABLE_PATH_RESTRICTIONS=true m
Change-Id: Id68cbb1c8ceef65bbbb10751e83722c7662d2351

python/scripts/stub_template_host.txt

@@ -18,8 +18,6 @@ def SearchPathEnv(name):
   for directory in search_path:
     if directory == '': continue
     path = os.path.join(directory, name)
-    if os.path.islink(path):
-      path = os.path.realpath(path)
     # Check if path is actual executable file.
     if os.path.isfile(path) and os.access(path, os.X_OK):
       return path