Use HOST_ANDROID define instead of PLATFORM_ANDROID

Use HOST_ANDROID define instead of PLATFORM_ANDROID
(a cherry-pick of commit 5d819c8 from 'mono_libgc')

* doc/README.macros (PLATFORM_ANDROID): Rename to HOST_ANDROID.
* dyn_load.c: Rename PLATFORM_ANDROID to HOST_ANDROID.
* include/gc.h: Likewise.
* include/gc_config_macros.h: Likewise.
* include/private/gcconfig.h: Likewise.
* include/private/thread_local_alloc.h: Likewise.
* os_dep.c: Likewise.
* pthread_support.c: Likewise.
* tests/initsecondarythread.c: Likewise.
* tests/test.c: Likewise.
* tools/threadlibs.c: Likewise.