mallocinit has evolved organically. Make a pass to clean it up in
various ways:
1. Merge the computation of spansSize and bitmapSize. These were
computed on every loop iteration of two different loops, but always
have the same value, which can be derived directly from _MaxMem.
This also avoids over-reserving these on MIPS, were _MaxArena32 is
larger than _MaxMem.
2. Remove the ulimit -v logic. It's been disabled for many releases
and the dead code paths to support it are even more wrong now than
they were when it was first disabled, since now we *must* reserve
spans and bitmaps for the full address space.
3. Make it clear that we're using a simple linear allocation to lay
out the spans, bitmap, and arena spaces. Previously there were a
lot of redundant pointer computations. Now we just bump p1 up as we
reserve the spaces.