Implement --execute-only (and turn --no-execute-only from a no-op into an

Implement --execute-only (and turn --no-execute-only from a no-op into an
options that does the opposite).

Note that this option is likely to be ineffective without changes to the
linker scripts.  A change that adjusts the hppa linker scripts will follow
shortly.  Other architectures will need some work.

ok deraadt@