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