user stack randomization: only enable by default for 64bit processes

[kib]

Summary

All aslr knobs are disabled by default for 32bit processes, except
stack.  This results in weird stack location, typically making around 1G
of user address space hard to use.

[emaste]

Yes, it makes sense to have this consistent with other 32/64-bit ASLR knobs.

The description in mitigations.7 and security.7 says If ASLR is enabled for a process, also randomize the stack location.(or equivalent) but it looks like that may be referring to an older implementation. I can update that text.