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)
This slightly reduces memory use.
Maximum memory use of `bun test html-rewriter`, averaged across 100
iterations:
* 101975 kB without this change
* 101634 kB with this change
I also tried changing the code to always use the aligned allocation
functions, but this slightly increased memory use, to 102160 kB.
(For internal tracking: fixes ENG-19866)
