rustc: Clean up allocator injection logic

rustc: Clean up allocator injection logic

This commit cleans up allocator injection logic found in the compiler
around selecting the global allocator. It turns out that now that
jemalloc is gone the compiler never actually injects anything! This
means that basically everything around loading crates here and there can
be easily pruned.

This also removes the `exe_allocation_crate` option from custom target
specs as it's no longer used by the compiler anywhere.