doc/changes.src: document massive changes to ELF64 backend
doc/changes.src: document massive changes to ELF64 backend

We didn't just fix the GOT stuff, but also now properly use RELA and
so forth.

Signed-off-by: H. Peter Anvin <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,techkey/nasm,letolabs/nasm,letolabs/nasm,projedi/nasm,projedi/nasm,turingstudio/nasm,techkey/nasm,projedi/nasm,Distrotech/nasm,projedi/nasm,turingstudio/nasm,turingstudio/nasm,techkey/nasm,Distrotech/nasm,letolabs/nasm,Distrotech/nasm,techkey/nasm,projedi/nasm,turingstudio/nasm,Distrotech/nasm