rollup merge of #19534: scialex/liballoc-cfg

Adds the ability to use a custom allocator heap by passing either --cfg
external_crate and --extern external=<allocator_crate_name> or --cfg
external_funcs and defining the allocator functions prefixed by 'rust_'
somewhere.

This is useful for many applications including OS/embedded development,
and allocator development and testing.