[PATCH] elf: Keep only one '@' for undefined versioned symbols

[Fangrui Song]

On 2021-08-01, Alan Modra via Binutils wrote:
>On Sat, Jul 31, 2021 at 09:00:45AM -0700, H.J. Lu wrote:
>> It matches nm behavior.
>
>Not true.  PR28158.

Interesting. Since the shared object definition is of a default version,
it makes sense for the copy relocation to be a default version (@@).

llvm-readelf & llvm-nm print one @. I will fix them.