We can't use `std.heap.c_allocator` as `z_allocator`; it doesn't
zero-initialize the memory. This PR adds a fallback implementation.
This PR also makes Bun compile successfully with `use_mimalloc` set to
false. More work is likely necessary to make it function correctly in
this case, but it should at least compile.
(For internal tracking: fixes STAB-978, STAB-979)
