[android] pthread_kill cannot be used safely on android

[android] pthread_kill cannot be used safely on android

Android's fork() function does not properly update the
pthread_internal_t->threadid variable to point to the tid of the
fork()'d thread, and instead still points to the pid of zygote.
As a result we need to track the android tid's manually since
gettid() is sane, and use an alternate to pthread_kill on android
Additionally, disable sgen support on android until the same
work has been done there

r: kumpera, vargaz