pie: Give VDSO symbol table local scope

In commit c2271198, Laurent Dufour kindly reunified the VDSO code
that had become duplicated between architectures. Unfortunately
this introduced a regression in AArch64 where apparently due to
the scope of vdso_symbols array of pointers to characters changing
from local to global, load-time relocations became necessary.

The following thread on the GCC mailing list discusses why
load-time relocations can be necessary when pointers are used,
although it doesn't mention the potential for locally scoped
arrays to be handled differently:
https://gcc.gnu.org/ml/gcc/2004-05/msg01016.html



Because the alternatives, such as porting piegen to AArch64, are
far more involved, simply revert the change in scope.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>