unix/mpthreadport: Ensure enough thread stack to detect overflow.
unix/mpthreadport: Ensure enough thread stack to detect overflow.

Following up to 5e6cee07aba4fd73c13024f091a287171bea1f17, some systems (eg
FreeBSD 12.0 64-bit) will crash if the stack-overflow margin is too small.
It seems the margin of 8192 bytes (or thereabouts) is always needed.  This
commit adds this much margin if the requested stack size is too small.

Fixes issue #5824.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython