Rollup merge of #114412 - RalfJung:libc-symbols, r=pnkfelix

document our assumptions about symbols provided by the libc

LLVM makes assumptions about `memcmp`, `memmove`, and `memset` that go beyond what the C standard guarantees -- see https://reviews.llvm.org/D86993. Since we use LLVM, we are inheriting these assumptions.

With rust-lang/rust#114382 we are also making a similar assumption about `memcmp`, so I added that to the list.

Fixes rust-lang/unsafe-code-guidelines#426.

Author: matthiaskrgr