Update AMD backend

Rename AMD to AMDSMI to make it clear that the AMD SMI library is used internally. This backend is updated to work better in systems with more than one GPU, and is tolerant to cases where clock frequencies can not be read (e.g. for integrated graphics). Additionally, a new backend RSMI is added that uses the (legacy) ROCM SMI library internally. In contrast to AMD SMI, ROCM SMI does support reading the clock frequencies for integrated graphics. Both AMDSMI and RSMI are built respectively when the user specifies FMT_BUILD_AMDSMI=1 and FMT_BUILD_RSMI. Both backends use the Pimpl structure to avoid exposing ROCM SMI or AMD SMI specific headers files in the public header files.