libpthread/nptl: make default stack size configurable

libpthread/nptl: make default stack size configurable

Threads currently have 2-4 MiB stacks by default (depending on the
platform). This is fine on MMU platforms, where this stack space is not
actually allocated until it is used, but tends to waste a large amount
of memory on no-MMU platforms.

This patch adds a PTHREADS_STACK_DEFAULT_SIZE Kconfig option that allows
the user to override the default stack size at build time. This allows
the user to select a reasonable default stack size for the software that
runs on their system, and avoids the need to patch every package to add
calls to pthread_attr_setstacksize().

An alternative to this patch would be to change the hardcoded default
stack size on no-MMU platforms, but it is difficult to choose an
appropriate value because the minimum required stack depends on the
software in use. This would also be a breaking change.

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>