Allow ROCm arch override

[ailuntz]

Fixes #1444.\n\nSupport BNB_ROCM_GPU_ARCH override when rocminfo is unavailable.

[Abdennacer-Badaoui]

Thanks @ailuntz.
A few things need adjustment: First, the override should be a fallback after rocminfo fails, not a priority override that runs before it; for most users, auto-detection is more reliable and shouldn't be bypassed. Second, use only BNB_ROCM_GPU_ARCH and drop BNB_ROCM_ARCH, that name is already used as a CMake build-time variable and reusing it at runtime with different semantics is confusing. Third, add some basic validation on the value (e.g. check it matches gfx[a-zA-Z0-9]+) so garbage input doesn't silently propagate. And finally, the existing error message should mention BNB_ROCM_GPU_ARCH as a workaround so users know what to do when rocminfo fails (ask users to install rocminfo, if it's not possible then ask them to set the env variable)