stack overflow handler specific openbsd fix.
stack overflow handler specific openbsd fix.

On this platform, when doing stack allocation, MAP_STACK is needed
 otherwise the mapping fails.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust