* Use BX_PLATFORM_POSIX where needed
The semaphone implementation already uses BX_PLATFORM_POSIX to include
<pthread.h> on all the POSIX platforms; do the same also in other places
for consistency. This is done also for <sched.h>, which is a POSIX API,
and sched_yield() from it is already guarded by BX_PLATFORM_POSIX.
* Drop support for GNU libc older than 2.12
glibc 2.12 was released on 2010, and at this point any supported Linux
distro has that version or way greather than that.