elf_common: add ELF note names

r348628 added a definition of NT_GNU_BUILD_ID. Some software (Valgrind)
also expects a #define for the note name (ELF_NOTE_GNU) in the case that
NT_GNU_BUILD_ID is defined.

PR: 239669
Reported by: Yuichiro NAITO
Sponsored by: The FreeBSD Foundation
Event: EuroBSDCon FreeBSD DevSummit 2019