[libc] [llvm] [libc] Adding a version of memset with software prefetching (PR #70857)

[Guillaume Chatelet via llvm-commits]

https://github.com/gchatelet commented:

Ok this looks pretty good to me. We also have to add support to CMake and make sure the option is discoverable. This means adding an entry in https://github.com/llvm/llvm-project/blob/main/libc/config/config.json. It should mimic the `LIBC_COPT_MEMSET_X86_USE_SOFTWARE_PREFETCHING` without the `COPT` part.
You can draw inspiration from https://github.com/llvm/llvm-project/commit/380eb46b13175aef5f98f0613bcc9207021efce0 (don't update libc/docs/configure.rst yourself, it's now automatically done from the json file).
For now the option should be set to `false`, once I've checked that the codegen looks good we'll enable the option by overriding it in `libc/config/linux/x86_64/config.json` and `libc/config/windows/x86_64/config.json`.

https://github.com/llvm/llvm-project/pull/70857