Workaround some Linux/arm kernels bug to get correct GC_nprocs value

Workaround some Linux/arm kernels bug to get correct GC_nprocs value

* pthread_support.c (STAT_READ, STAT_BUF_SIZE): Move the definition
out of PLATFORM_ANDROID condition.
* pthread_support.c (GC_get_nprocs_present): New static function
obtaining number of avalable CPUs from
"/sys/devices/system/cpu/present" (only Linux/arm excluding NaCl).
* pthread_support.c (GC_thr_init): Invoke GC_get_nprocs_present
(unless set from the environment variable) to workaround a bug in
some Linux/arm kernels (including Android).