Port std::arch test suite to rust-lang/rust

The std::arch in the standard library provides architecture-specific intrinsic functions, which directly map to a single machine instruction. The ultimate goal is to inline stdarch into rustc completely and archive the stdarch repository making stdarch tests reliable and maintainable inside rustc infrastructure. Which we will achieve by incrementally investigating the CI test suite of stdarch and port as much of it into rust-lang/rust. The clear set of deliverables for this project are Map stdarch CI to rustc, land x.py steps for verify + stdarch tests, enable core_arch (plain then assert_instr on Linux), port intrinsic-test in phases (host, then cross if feasible), minimal stdarch-gen checks, wire CI, document, final report.