Rollup merge of #145954 - RalfJung:syscall-c-variadics, r=jackh726

stabilize extern_system_varargs

Based on top of rust-lang/rust#144066. This has been already FCP'd over there, but `@workingjubilee` has some concerns regarding "system" varargs specifically (IIUC).

Reference PR: rust-lang/reference#2069.

Author: Zalathar