posix: pthread: support stack sizes larger than 65k

A previous size optimization capped the pthread_attr_t stacksize
property at 65536. Some Zephyr users felt that was not large
enough for specific use cases.

Modify struct pthread_attr to support large stack sizes by
default with the flexibility to allow users to vary the number
of bits used for both stacksizes and guardsizes.

The default guardsize remains zero sinze Zephyr's stack
allocators already pad stacks with a guard area based on other
config parameters, and since Zephyr is already designed to
support both SW and HW stack protection at the kernel layer.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>