fix build regression on archs with variable page size

fix build regression on archs with variable page size

commit 31fb174dd295e50f7c5cf18d31fcfd5fe5a063b7 used
DEFAULT_GUARD_SIZE from pthread_impl.h in a static initializer,
breaking build on archs where its definition, PAGE_SIZE, is not a
constant. instead, just define DEFAULT_GUARD_SIZE as 4096, the minimal
page size on any arch we support. pthread_create rounds up to whole
pages anyway, so defining it to 1 would also work, but a moderately
meaningful value is nicer to programs that use
pthread_attr_getguardsize on default-initialized attribute objects.