MinGW: Restore using windres when toolchain-prefixed name is not available

Since commit 55ba10dcfd (MSYS/MinGW Makefiles: Simplify selection of
windres as Resource Compiler, 2022-05-26, v3.24.0-rc1~82^2) the `MinGW
Makefiles` and `MSYS Makefiles` generators no longer specify the plain
`windres` name for the MinGW resource compiler.  Instead, the name is
specified in our MinGW platform information module. After the change in
commit af4adf6aa9 (MinGW: Fix default windres selection when
cross-compiling, 2020-03-25, v3.18.0-rc1~492^2), when cross-compiling,
we only specify the toolchain-prefixed name of the tool, which may not
be available in all environments.

If the toolchain-prefixed name is not available, fall back to the plain
`windres` name.  We already use this approach for other binutils.

Fixes: #23841