riscv: spike: allow base == 0

riscv: spike: allow base == 0

The sanity check on base doesn't allow htif to be mapped @0. Check if the
symbol exists instead so we can map it where we want.

Reviewed-by: Michael Clark <mjc@sifive.com>
Signed-off-by: KONRAD Frederic <frederic.konrad@adacore.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Message-Id: <1525360636-18229-2-git-send-email-frederic.konrad@adacore.com>
(cherry picked from commit 17b9751e85b9989cc841ed387794d7f1e8aa5e46)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>