pthread_md.h. This commit only moves the definition; it does not
change it for any of the platforms. This more easily allows 64-bit
architectures (in particular) to pick a slightly larger stack size.
archs that can (or are required to) have per-thread registers.
Tested on i386, amd64; marcel is testing on ia64 and will
have some follow-up commits.
Reviewed by: davidxu