Set CMAKE_NM in CMake toolchain file

[croes]

Set CMAKE_NM in the CMake toolchain file.

We ran into some issues where the system's nm was used instead of emnm, and we traced it back to the Emscripten CMake toolchain file not setting CMAKE_NM.

We fixed this locally by setting CMAKE_NM after including the Emscripten toolchain CMake file, but figured it's better to push a fix upstream.

[croes]

After investigating further, the underlying issue was that the Emscripten CMake toolchain file wasn't correctly passed when compiling some third-party dependencies (we use conan, and had to define tools.cmake.cmaketoolchain:user_toolchain for dependencies using the CMakeToolchain template). This caused linking / nm issues later on.

Defining CMAKE_NM still might make sense (there's little to no documentation on this), but it's also fine to just close this PR. In the end, it wasn't the cause of our issue.