[scudo][standalone] Introduce the C & C++ wrappers [fixed]

Summary:
This is a redo of D63612.

Two problems came up on some bots:
- `__builtin_umull_overflow` was not declared. This is likely due to an
  older clang or gcc, so add a guard with `__has_builtin` and fallback
  to a division in the event the builtin doesn't exist;
- contradicting definition for `malloc`, etc. This is AFAIU due to the
  fact that we ended up transitively including `stdlib.h` in the `.inc`
  due to it being the flags parser header: so move the include to the
  cc instead.

This should fix the issues, but since those didn't come up in my local
tests it's mostly guesswork.

Rest is the same!

Reviewers: morehouse, hctim, eugenis, vitalybuka, dyung, hans

Reviewed By: morehouse, dyung, hans

Subscribers: srhines, mgorny, delcypher, jfb, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63831

llvm-svn: 364547