OpenHarmony (OHOS) is now recognized as an Android subsystem

Meson now recognizes OpenHarmony / HarmonyOS (OHOS). OHOS behaves like Android -- applications are shared libraries, shared libraries are not versioned, and so on -- but uses musl instead of Bionic. It is therefore modelled as an Android subsystem rather than a separate system.

Machine files select it in the [host_machine] section with:

[host_machine]
system = 'android'
subsystem = 'ohos'

With this, host_machine.system() is 'android' (so all Android handling applies, including unversioned libfoo.so output) while host_machine.subsystem() is 'ohos'. When cross-compiling a CMake subproject, the toolchain file generated by the CMake module sets CMAKE_SYSTEM_NAME = OHOS.