HOST_CROSS_OS/ARCH can be configured to linux_bionic/arm64

Previously, HOST_CROSS_OS/ARCH were fixed to windows/x86. This change
makes the setting configuration and adds the support for new OS/ARCH
combo: linux_bionic/arm64.

linux_bionic is the Linux-based host target that uses Bionic (instead of
glibc) as libc. Previously, it supported only x86_64 and the x86_64
target was NOT configured via Make, but directly via editing
soong.variables file. Now, the support for arm64 is being added in the
Soong side and this change makes it possible to configure the target via
Make.

The new HOST_CROSS_OS/ARCH combo will be used for building the host-side
tools (adb, crosvm, etc.) for running Cuttlefish natively on Linux/ARM
hosts.

Bug: 159685774
Test: HOST_CROSS_OS=linux_bionic HOST_CROSS_ARCH=arm64 m nothing

Change-Id: I6b8ed8f7e26908749bbe778fbdcc34cfbde68179

core/combo/HOST_CROSS_linux_bionic-arm64.mk

@@ -0,0 +1,22 @@
+#
+# Copyright (C) 2020 The Android Open Source Project
+#
+# 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.
+#
+
+# Configuration for builds hosted on linux_arm-arm64
+# Included by combo/select.mk
+
+define $(combo_var_prefix)transform-shared-lib-to-toc
+$(call _gen_toc_command_for_elf,$(1),$(2))
+endef

core/envsetup.mk

@@ -148,15 +148,25 @@ HOST_OS_EXTRA := $(subst $(space),-,$(HOST_OS_EXTRA))
 # BUILD_OS is the real host doing the build.
 BUILD_OS := $(HOST_OS)
 
-HOST_CROSS_OS :=
+# We can do the cross-build only on Linux
-# We can cross-build Windows binaries on Linux
 ifeq ($(HOST_OS),linux)
-ifeq ($(BUILD_HOST_static),)
+  # Windows has been the default host_cross OS
-HOST_CROSS_OS := windows
+  ifeq (,$(filter-out windows,$(HOST_CROSS_OS)))
-HOST_CROSS_ARCH := x86
+    # We can only create static host binaries for Linux, so if static host
-HOST_CROSS_2ND_ARCH := x86_64
+    # binaries are requested, turn off Windows cross-builds.
-2ND_HOST_CROSS_IS_64_BIT := true
+    ifeq ($(BUILD_HOST_static),)
-endif
+      HOST_CROSS_OS := windows
+      HOST_CROSS_ARCH := x86
+      HOST_CROSS_2ND_ARCH := x86_64
+      2ND_HOST_CROSS_IS_64_BIT := true
+    endif
+  else ifeq ($(HOST_CROSS_OS),linux_bionic)
+    ifeq (,$(HOST_CROSS_ARCH))
+      $(error HOST_CROSS_ARCH missing.)
+    endif
+  else
+    $(error Unsupported HOST_CROSS_OS $(HOST_CROSS_OS))
+  endif
 endif
 
 ifeq ($(HOST_OS),)

core/main.mk

@@ -606,8 +606,8 @@ $(strip \
   $(eval modules_32 := $(patsubst %:32,%,$(filter %:32,$(2)))) \
   $(eval modules_64 := $(patsubst %:64,%,$(filter %:64,$(2)))) \
   $(eval modules_both := $(filter-out %:32 %:64,$(2))) \
-  $(eval ### For host cross modules, the primary arch is windows x86 and secondary is x86_64) \
+  $(eval ### if 2ND_HOST_CROSS_IS_64_BIT, then primary/secondary are reversed for HOST_CROSS modules) \
-  $(if $(filter HOST_CROSS,$(1)), \
+  $(if $(filter HOST_CROSS_true,$(1)_$(2ND_HOST_CROSS_IS_64_BIT)), \
     $(eval modules_1st_arch := $(modules_32)) \
     $(eval modules_2nd_arch := $(modules_64)), \
     $(eval modules_1st_arch := $(modules_64)) \