use @@@ instead of @@ in __sym_default

Using

.symver foo,foo@@VER

causes foo and foo@@VER to be output to the .o file. This requires foo
to be weak since the linker handles foo@@VER as foo.

Using

.symver foo,foo@@@VER

causes just foo@@ver to be output and avoid the need for making foo
weak. It also reduces the constraint on how exactly a linker has to
handle foo and foo@@VER being present.

Submitted by: Rafael Espíndola
Reviewed by: dim, kib
Differential Revision: https://reviews.freebsd.org/D11653