[AMDGPU] Introduce Code Object V6 (#76954)

Introduce Code Object V6 in Clang, LLD, Flang and LLVM. This is the same
as V5 except a new "generic version" flag can be present in EFLAGS. This
is related to new generic targets that'll be added in a follow-up patch.
It's also likely V6 will have new changes (possibly new metadata
entries) added later.

Docs change are part of the follow-up patch #76955