ASM: Restore CMAKE_{C,CXX}_COMPILER as ASM-first candidates

Prior to commit abfe98ce6c (ASM: Consider C/CXX compilers only if
enabled, 2025-04-08) one could pass `-DCMAKE_C_COMPILER=...` to
configure a project that enables `ASM` before/without `C` or `CXX`.
This was previously allowed accidentally as a side effect of the way we
implemented support for using the selected `C` or `CXX` compiler for
`ASM`.  Restore it for compatibility, but also preserve the effect of
commit 6baf65ec46 (ASM: Do not consider MSVC C/CXX compiler for generic
ASM, 2025-04-08) by excluding `cl`.