lib/core/fiber: Increase default stack size

The default 64K stack size used for years become too small
for modern distors (Fedora 29 and etc) where third party libraries
(such as ncurses) started to use 64K for own buffers and we get
SIGSGV early without reaching interactive console phase.

Thus we increase default size up to 512K which should fit
for common case. Later we will make this value configurable
to address arbitrary stack sizes without a need to rebuild
the whole code.

Closes #3418