llvm-rc: Fix regression on .rc sources with LLVM/Clang tooling

Refactoring in commit 2b2344b412 (MSVC: Add abstraction for runtime
checks, 2025-01-22) switched to using local compiler id variables to
recognize compilers that target the MSVC ABI.  However, those variables
may be modified modified in a special case for `lang == "RC"` that does
not apply to the ABI check.  Check the target ABI before applying the
special case.

Fixes: #26663