ELF: Properly redefine PT_GNU_* in terms of PT_LOOS

The PT_GNU_* program header types are actually offsets from PT_LOOS,
so redefine them as such, reorder them, and add the missing PT_GNU_RELRO.

Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>