[scudo] Overhaul malloc related interceptors

Summary:
This is a follow-up to D42506.

There are a few of things that bothered me about `scudo_interceptors.cpp`:
- the filename is a misnomer: it intercepts some functions, but the rest (C++)
  is actually in `scudo_new_delete.cpp`. I feel like `scudo_malloc.cpp` is more
  appropriate (ASan uses the same naming scheme);
- we do not need "full" interceptors, since we are never accessing the
  unsanitized version of the functions, we just need the
  `extern "C" INTERCEPTOR_ATTRIBUTE` part of it to just call our functions;
- a couple of functions where duplicated while they could just be `ALIAS`'d;
- use the `SANITIZER_INTERCEPT_*` defines to hide the unneeded interceptors;
- use `SIZE_T` instead of `uptr`: while it's the same behind the curtain,
  the former is meant for this use case.

In the end there is no functional change on the currently supported platforms
(Linux, Android).

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: mgorny, hintonda, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42546

llvm-svn: 323464