Provide a function in envsetup.sh to get the host arch directly

It's very simple logic, and running get_build_var to do it is slow
and has side effects (like needing a lunch target, creating an out
directory, etc.)

Test: source envsetup.sh
Change-Id: If260efd21713874fba7c15dbc0fd23442d776f8a

core/envsetup.mk

diff --git a/core/envsetup.mk b/core/envsetup.mk
index 1c3a1b3..3271079 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -255,6 +255,7 @@
 HOST_PREBUILT_ARCH := x86
 # This is the standard way to name a directory containing prebuilt host
 # objects. E.g., prebuilt/$(HOST_PREBUILT_TAG)/cc
+# This must match the logic in get_host_prebuilt_prefix in envsetup.sh
 HOST_PREBUILT_TAG := $(BUILD_OS)-$(HOST_PREBUILT_ARCH)
 
 # TARGET_COPY_OUT_* are all relative to the staging directory, ie PRODUCT_OUT.

envsetup.sh

diff --git a/envsetup.sh b/envsetup.sh
index 50fec51..ff3c3a6 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -196,6 +196,19 @@
     (\cd $T; build/soong/soong_ui.bash --dumpvar-mode $1)
 }
 
+# This logic matches envsetup.mk
+function get_host_prebuilt_prefix
+{
+  local un=$(uname)
+  if [[ $un == "Linux" ]] ; then
+    echo linux-x86
+  elif [[ $un == "Darwin" ]] ; then
+    echo darwin-x86
+  else
+    echo "Error: Invalid host operating system: $un" 1>&2
+  fi
+}
+
 # check to see if the supplied product is one we can build
 function check_product()
 {