Fix rocWMMA build documentation

[Headcrabed]

Current HIP build manual about rocWMMA actually not working under linux, so here's the fix for this.

[Headcrabed]

@ggerganov ping

[IMbackK]

I don't think this has anything to do with windows vs Linux but rather with compiling with hipcc vs compiling with plain clang as cxx compiler. Regardless I think CMAKE_HIP_FLAGS should work in all cases, but this would have to test than on windows, which I don't have, but I will try doing so on ci soon

[Headcrabed]

I don't think this has anything to do with windows vs Linux but rather with compiling with hipcc vs compiling with plain clang as cxx compiler. Regardless I think CMAKE_HIP_FLAGS should work in all cases, but this would have to test than on windows, which I don't have, but I will try doing so on ci soon

@IMbackK Currently DCMAKE_CXX_FLAGS won't work in linux, but I don't have a test environment to try DCMAKE_HIP_FLAGS in windows. 🙃