Define PTHREAD_STACK_MIN to sysconf(_SC_THREAD_STACK_MIN)

The constant PTHREAD_STACK_MIN may be too small for some processors.
Rename _SC_SIGSTKSZ_SOURCE to _DYNAMIC_STACK_SIZE_SOURCE.  When
_DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE are defined, define
PTHREAD_STACK_MIN to sysconf(_SC_THREAD_STACK_MIN) which is changed
to MIN (PTHREAD_STACK_MIN, sysconf(_SC_MINSIGSTKSZ)).

Consolidate <bits/local_lim.h> with <bits/pthread_stack_min.h> to
provide a constant target specific PTHREAD_STACK_MIN value.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>