targets/xa: limit the amount of exported symbols

In the presence of LLVM the final library exports every symbol from
the llvm namespace. Resolve this by using a version script (w/o the
version/name tag).

Considering that there are only ~35 symbols, explicitly list them
to minimize the chances of rogue symbols sneaking in.

v2: Conditionally include the version-script.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com> (v1)
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
(cherry picked from commit a75baba2f14a2bcdaf640aeef86d67b41bb2a76f)
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>