add pthread_attr_setstack interface (and get)

i originally omitted these (optional, per POSIX) interfaces because i
considered them backwards implementation details. however, someone
later brought to my attention a fairly legitimate use case: allocating
thread stacks in memory that's setup for sharing and/or fast transfer
between CPU and GPU so that the thread can move data to a GPU directly
from automatic-storage buffers without having to go through additional
buffer copies.

perhaps there are other situations in which these interfaces are
useful too.

src/thread/pthread_attr_getstack.c

@@ -0,0 +1,10 @@
+#include "pthread_impl.h"
+
+int pthread_attr_getstack(const pthread_attr_t *a, void **addr, size_t *size)
+{
+	if (!a->_a_stackaddr)
+		return EINVAL;
+	*size = a->_a_stacksize + DEFAULT_STACK_SIZE;
+	*addr = (void *)(a->_a_stackaddr - *size);
+	return 0;
+}