Add MMAllocMisc / MMFreeMisc and use them.

We've long had a couple of hacks to permit using GC_MALLOC / GC_FREE
from within Dylan code. This begins to remove those hacks. The other
usages need additional work to remove.
Use MMAllocMisc/MMFreeMisc for envp.

We create an environment data block for passing to spawn() but we
need to use MMAllocMisc and MMFreeMisc for managing it rather than
GC_malloc.
Remove (now) unused hack for GC_malloc.
Merge pull request #470 from waywardmonkeys/remove-gc-malloc-hacks

Remove gc malloc hacks