[llvm][DebugInfo][test] dwarf-asm-multiple-sections.s: refine FileCheck checks
Renames the `DWARF6` check-prefix to `DWARF7` because it's the check we use for `-dwarf-version 7`.
Also adds a check prefix for just `-dwarf-version 5` because we will
diverge DWARFv5 and DWARFv6 CU langauge attribute in https://github.com/llvm/llvm-project/pull/183897.