Adding a macro to define the vlog module in use adds a level of
indirection, which makes it easier to change how the vlog module must be
defined. A followup commit needs to do that, so getting these widespread
changes out of the way first should make that commit easier to review.
The portable implementation of stack_low(), which before this commit is
used on x86-64, provokes a warning from GCC that cannot be disabled. We
already have an i386-specific implementation that does not warn; this
commit adds a corresponding implementation for x86-64 to avoid the warning
there too.
Without this change GCC warns "use of assignment suppression and length
modifier together in scanf format", which doesn't actually point out any
real problem (and why would it? Google turns up nothing interesting).
