`c_variadic`: impl `va_copy` and `va_end` as Rust intrinsics
tracking issue: https://github.com/rust-lang/rust/issues/44930
Implement `va_copy` as (the rust equivalent of) `memcpy`, which is the behavior of all current LLVM targets. By providing our own implementation, we can guarantee its behavior. These guarantees are important for implementing c-variadics in e.g. const-eval.
Discussed in [#t-compiler/const-eval > c-variadics in const-eval](https://rust-lang.zulipchat.com/#narrow/channel/146212-t-compiler.2Fconst-eval/topic/c-variadics.20in.20const-eval/with/565509704).
I've also updated the comment for `Drop` a bit. The background here is that the C standard requires that `va_end` is used in the same function (and really, in the same scope) as the corresponding `va_start` or `va_copy`. That is because historically `va_start` would start a scope, which `va_end` would then close. e.g.
https://softwarepreservation.computerhistory.org/c_plus_plus/cfront/release_3.0.3/source/incl-master/proto-headers/stdarg.sol
```c
#define va_start(ap, parmN) {\
va_buf _va;\
_vastart(ap = (va_list)_va, (char *)&parmN + sizeof parmN)
#define va_end(ap) }
#define va_arg(ap, mode) *((mode *)_vaarg(ap, sizeof (mode)))
```
The C standard still has to consider such implementations, but for Rust they are irrelevant. Hence we can use `Clone` for `va_copy` and `Drop` for `va_end`.
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