NMake: Use UTF-8 BOM in response files only with MSVC tooling

Since commit f3f57cc4 (NMake: Use UTF-8 with BOM if supported by
nmake, 2021-04-22, v3.21.0-rc1~217^2), we add a BOM to response files
to tell MSVC tooling that they are encoded as UTF-8.  However, the
"NMake Makefiles" generator may also be used with non-MSVC toolchains
that do not understand the BOM.  Update the response file encoding
selection heuristic to add the BOM only with MSVC tooling.

Fixes: #23143