Add ET_GNU_DEBUG
We need a way to identify "debug" info files, which appear like they are ELF files but if inspected are actually missing a lot of information and can't be properly parsed without the original DSO or executable. We propose
#define ET_GNU_DEBUG 0xfe00 /* GNU debug information file */
A ET_GNU_DEBUG file may contain the same program headers as the associated (original) object file, but all the loadable segments are missing from the ET_GNU_DEBUG file (including the dynamic segment). Consumers may skip ET_GNU_DEBUG files. Debuggers can extract debug information of the associated object file from the ET_GNU_DEBUG file.