libc: Add stack protector support back

Based on regressions indicated by dports bulk build (< 10), it appears
that the deactivated stack_protector.c code was used with static stack
protection (e.g.

libc: Add stack protector support back

Based on regressions indicated by dports bulk build (< 10), it appears
that the deactivated stack_protector.c code was used with static stack
protection (e.g. -lssp_nonshared).

FreeBSD moved the file to its own area less than a month ago, which is
why I thought it had been removed. The plan is to put all security
related code in the same area, so I'm going to follow suit. While here,
remove the stack_smash_handler, which I believe is obsolete, and reduce
differences with FreeBSD.

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