Document supported OS, architectures, and compilers

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mimalloc's README does not contain a list of supported operating systems, CPU architectures, and compilers. For integration with CPython, it would be helpful to document which combinations of OS, CPU, and compiler are officially supported or at least work. Our users run Python on a lot of platforms, some of them are pretty esoteric.

Just to be clear, we do **not** expect that you add support for all these platforms yourself. For now we want to get a better understanding of support platforms.

I digged into the source code and ran mimalloc through Python buildbot farm. I got mimalloc working on majority of buildbots. The failing builds on some RHEL 7 boxes is caused by outdated GCC. There also seems to be an issue on s390x with one test case that consumes a lot of memory. I'll look into the failing test at a later point in time.

## compilers

* GCC >= 4.9
* clang >= 3.6
* MSVC >= 2012 (11.0)
* ICC (successfully tested with 2021.5, earlier might work)
* any other C99 compiler with C11 ``stdatomic.h`` or C++11 compiler

## platforms

* Windows
* X86 ``_M_IX86``
* X86_64 ``_M_X64``
* ARM64 ``_M_ARM64``
* Linux
* X86_64 ``__x86_64__``
* i386 ``__i386__``
* ARM ``__arm__`` (minimum ``__ARM_ARCH__``?)
* ARM little endian ``__ARMEL__``
* aarch64 ``__aarch64__`` (ARM 64bit, LE and BE?)
* PowerPC / PowerPC 64 (``__powerpc__``, ``__ppc__``)
* s390x (not mentioned, seem to work out of the box)
* more?
* macOS (Darwin)
* X86_64
* M1 (still work in progress?)
* Android (which archs?)
* FreeBSD, NetBSD, OpenBSD (which archs?)
* DragonFly (still buggy?)

## Unknown

* MIPS (MIPSEL, MIPS64EL)
* m86k
* PPC64 / PPC64EL (covered by ``__ppc__`` ?)
* Solaris (Illumos, OpenIndiana), with default CC
* HP-UX, with HP C compiler
* AIX with xlC compiler
* Wind River VxWorks

## Unsupported

AIX xlc compiler (13.1, 16) does not support ``stdatomic.h``. It has [GCC atomic memory access built-in functions](https://www.ibm.com/docs/en/xl-c-aix/13.1.3?topic=cbif-gcc-atomic-memory-access-built-in-functions-extension).

Start with the README and the source code's platform and compiler checks, then compare them with the Python buildbot results described in the issue. Done means the README clearly documents supported or tested operating-system, architecture, and compiler combinations, while separating unknown and unsupported cases.

Written by the indexing model from the issue text.