initial port of the exchange allocator to jemalloc

In stage0, all allocations are 8-byte aligned. Passing a size and
alignment to free is not yet implemented everywhere (0 size and 8 align
are used as placeholders). Fixing this is part of #13994.

Closes #13616
fix Vec<ZeroSizeType>
gitattributes: rm obsolete jemalloc entry
kick buildbot