Make core::rt::rt_exchange_malloc's signature consistent with private::exchange_alloc::malloc's signature

[catamorphism]

As per XXX in core::rt

[yichoi]

path has changed

libstd/unstable/lang.rs
pub unsafe fn exchange_malloc(td: *c_char, size: uintptr_t) -> *c_char {}

rt/rust_exchange_alloc.cpp
void *rust_exchange_alloc::malloc(size_t size)

[thestinger]

The C++ exchange allocator is on the way out so this isn't necessary and can be closed.

Although, #7651 will bring the signature in line with C++ anyway.