[CUDA] Add CUDA fatbinary magic

Nvidia uses fatbinaries to bundle all of their device code. This patch
adds the magic number "0x50ed55ba" used in their propeitary format to
the list of magic identifies. This is technically undocumented and could
unlikely be changed by Nvidia in the future.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D120932