Account for the extra stack size when rlimit is small enough

Account for the extra stack size when rlimit is small enough

When rlimit is small enough to be used as the stacksize to be returned
in pthread_getattr_np, cases where a stack is made executable due to a
DSO load get stack size that is larger than what the kernel
allows. This is because in such a case the stack size does not account
for the pages that have auxv and program arguments.

Additionally, the stacksize for the process derived from this should
be truncated to align to page size to avoid going beyond rlimit.