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