[clang] [Docs] Document freestanding requirements (PR #132232)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

-ffreestanding currently has the following effects:
- Disables builtins recognition (-fno-builtins)
- Sets `__STDC_HOSTED__` to 0.
- Disables unwind tables (-fno-asynchronous-unwind-tables -fno-unwind-tables) (refer to df50259f9856ae98b5a1b34c4c955074aff3c566)

Maybe we should explicitly list this out in the documentation.

We might want to check existing uses of `__STDC_HOSTED__` to see if we need to modify the interaction if we have a libc.  A few of the headers in clang/lib/Headers/ make assumptions about the presence of libc based on `__STDC_HOSTED__`.  At first glance, we're probably fine, but we might want to come up with some sort of policy.

As a practical matter, nobody really cares what the C committee has to say about freestanding implementations; everyone makes different tradeoffs depending on the constraints of their environment.  Some "baremetal" environments have an extremely complete C library including I/O, and the usual "main" function.  Some have absolutely nothing (even less than what we say clang requires).  And everything in between.

I don't think we need to say anything about how startup works in a freestanding environment; I think we just refer people to their environment's documentation.

We say "memcpy, memmove, memset, memcmp" because that's what the corresponding gcc documentation says, but I don't think clang has ever generated calls to memcmp in -fno-builtins mode.

https://github.com/llvm/llvm-project/pull/132232