sysconf add param _SC_THREAD_STACK_MIN deal
make the pthread_exit pass the ltp/open_posix_testsuite/pthread_exit/1-2.c testcase
(1) in pthread_exit/1-2.c testcase, if sysconf(_SC_THREAD_STACK_MIN) % sysconf(_SC_PAGESIZE) is not zero, will report error and test failed, so set CONFIG_PTHREAD_STACK_MIN as DEFAULT_MM_PGSIZE`s default value.
(2) in function sysconf add param _SC_THREAD_STACK_MIN deal.
Signed-off-by: yanghuatao <yanghuatao@xiaomi.com>