Base max_fast on alignment, not width, of bins (Bug 24903)

set_max_fast sets the "impossibly small" value based on,
eventually, MALLOC_ALIGNMENT.  The comparisons for the smallest
chunk used is, eventually, MIN_CHUNK_SIZE.  Note that i386
is the only platform where these are the same, so a smallest
chunk *would* be put in a no-fastbins fastbin.

This change calculates the "impossibly small" value
based on MIN_CHUNK_SIZE instead, so that we can know it will
always be impossibly small.