Fix get_amd_offload_arch_flag so it will match offload-arch types having alphanumeric names.

[rasmith]

This PR changes the the get_amd_offload_arch_flag() function to match all offload-arch types that have alphanumeric names. For example, on MI250, the offload-arch is gfx90a. On the MI250a, the function did not match name correctly and passed down "gfx90" into hipcc, and hipcc returned an error.