stdlib: Fixing test-*atexit*-race tests on ia64

These tests require a new thread stack size set to a value (0x20000)
lower than the architecture minimum (0x30000).  Set the stack size
to PTHREAD_STACK_MIN in this case.

Checked on ia64-linux-gnu.

	* stdlib/test-atexit-race-common.c (do_test): Check stack size
	against PTHREAD_STACK_MIN.