[libc][docs] codify Policy on Assembler Sources (#88185)

It would be helpful in future code reviews to document a policy with
regards to
where and when Assembler sources are appropriate. That way when
reviewers point
out infractions, they can point to this written policy, which may help
contributors understand that it's not solely the personal preferences of
individual reviewers but instead rather a previously agreed upon rule by
maintainers.

Link: https://github.com/llvm/llvm-project/pull/87837
Link: https://github.com/llvm/llvm-project/pull/88157
Link:
https://discourse.llvm.org/t/hand-written-in-assembly-in-libc-setjmp-longjmp/73249/12