POC: add snmalloc as an alternative allocator to libc

[SchrodingerZhu]

Does not work yet. I need to work out a lot more details. This is for collecting ideas.

snmalloc is already supported in LLVM for windows CRT replacement, we can also use it as an implementation of our allocator:

llvm-project/llvm/lib/Support/CMakeLists.txt

Line 101 in aa0191e

if(LLVM_INTEGRATED_CRT_ALLOC)

Adding snmalloc will push forward our support on windows/macos easily. It also provides linux target with a modern allocator alternative.

I have made snmalloc buildable with clean C headers in microsoft/snmalloc#722, hence, it can now be used in a way largely similar to scudo standalone allocator.

malloc.cpp.o is already buildable with some ad-hoc fixes, so I think there is no technical obstacle.

[SchrodingerZhu]

cc @mjp41