Test fixes for Arm64EC Windows * `tests/ui/cfg/conditional-compile-arch.rs` needs an Arm64EC case. * `tests/ui/runtime/backtrace-debuginfo.rs` should skip Arm64EC as it suffers from the same truncated backtraces as Arm64 Windows. * `tests/ui/linkage-attr/incompatible-flavor.rs` is a general issue: it assumes that the Rust compiler is always built with the x86 target enabled in the backend, but I only enabled AArch64 when building locally to speed up the LLVM compilation.