RFC: Update MINSIGSTKSZ and SIGSTKSZ

[Paul Eggert]

On 9/29/20 2:07 PM, H.J. Lu wrote:
> Currently, SIGSTKSZ is 8KB.  We will have CPUs which require more than 8KB
> signal stake size in the very near future.

Then ports for those CPUs should increase SIGSTKSZ to a bigger constant now. 
That will be less work for everyone concerned than changing SIGSTKSZ to a 
non-constant expression, which violates POSIX and breaks too many programs.