LinuxBionic supports arm64

LinuxBionic now supports arm64 architecture in addition to the existing
x86_64 arch. This is to make it possible to build host tools like adb,
fastboot, crosvm, etc. for Linux/ARM on regular Linux/x86 machines.

The arm64 target can be selected in various ways in Android.bp files:

   - target.host (because this is still considered as a host target)
   - target.linux (provided that the module is also enabled for host)
   - target.linux_bionic (use the OS name directly)
   - target.linux_bionic_arm64 (OS name + arch combo)
   - target.linux_arm64 (provided that the module is also for host)
   - target.not_windows
   - arch.arm64

The toolchain for the new target is almost the same as the toolchain
config for Android/ARM64. One notable difference is that the clang
triple is aarch64-linux instead of aarch64-linux-android, so that
__ANDROID__ is not defined for the new OS type.

Bug: 134795810
Test: HOST_CROSS_OS=linux_bionic HOST_CROSS_ARCH=arm64 m nothing

Change-Id: If4300583edfb6d75bd4d984b38f73b6a406b4447
diff --git a/rust/config/arm64_device.go b/rust/config/arm64_device.go
index 180fd8b..b1f1f16 100644
--- a/rust/config/arm64_device.go
+++ b/rust/config/arm64_device.go
@@ -75,9 +75,16 @@
 }
 
 func Arm64ToolchainFactory(arch android.Arch) Toolchain {
+	archVariant := arch.ArchVariant
+	if archVariant == "" {
+		// arch variants defaults to armv8-a. This is mostly for
+		// the host target which borrows toolchain configs from here.
+		archVariant = "armv8-a"
+	}
+
 	toolchainRustFlags := []string{
 		"${config.Arm64ToolchainRustFlags}",
-		"${config.Arm64" + arch.ArchVariant + "VariantRustFlags}",
+		"${config.Arm64" + archVariant + "VariantRustFlags}",
 	}
 
 	toolchainRustFlags = append(toolchainRustFlags, deviceGlobalRustFlags...)