of more than 45G with a key_cache_block_size of 1024 or less.

The problem was that some of the arguments to my_multi_malloc() got to be
more than 4G.

Fix:
- Inntroduced my_multi_malloc_large() that can handle big regions.
- Changed MyISAM and Aria key caches to use my_multi_malloc_large().

I didn't change the default my_multi_malloc() as this would be a too big
patch and we don't allocate 4G blocks anywhere else.