Bring arch.h in line with upstream webrtc

Largely to bring in preprocessor support for additional architectures as based on 6215ba804eb500f3e28b39088c73af3c4f4cd10a by Timothy Gu timothygu99@gmail.com:

Add preprocessor support for additional architectures

• _M_ARM is used by Microsoft [1]
• __riscv and __riscv_xlen are defined by [2]
• __sparc and sparc are documented at [3]
• MIPSEB, PPC, PPC64 are documented at [3] and used in Chromium's build/build_config.h [4] Note: Chromium assumes that all PowerPC architectures are 64-bit. This is in fact not true.

[1]: https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-160
[2]: https://github.com/riscv/riscv-toolchain-conventions/tree/feca4793566811993f67e008a449d794cfc48953#cc-preprocessor-definitions
[3]: https://sourceforge.net/p/predef/wiki/Architectures/
[4]: https://source.chromium.org/chromium/chromium/src/+/master:build/build_config.h;drc=e12bf2e5ff1eacb9aca3e9a26bdeebdbdad5965a