Correct the default initial thread stack size.

[brooks]

Summary

By default, non-initial threads get 4MB stacks and initial ones get
twice that. There is no longer are 32-bit/64-bit split.

[kib]

Are you sure ? I do not see what you claim, in the code:

#define THR_STACK_DEFAULT		(sizeof(void *) / 4 * 1024 * 1024)

[brooks]

Bah, I did indeed misread the code.