[dmd-internals] Consistency with MODEL

[Jonathan M Davis]

I would point out that both druntime and phobos use 32 and 64 for the values 
of MODEL for building their 32-bit and 64-bit versions respectively, whereas 
dmd uses -m32 and -m64. Unless there is a good reason for dmd using -m32 and -
m64 instead of 32 and 64, it would probably be best (for consistency's sake, 
if nothing else), for dmd to use 32 and 64 for MODEL like druntime and phobos 
do.

- Jonathan M Davis