ld: xfail riscv64*-*-* for ld-scripts/empty-address-2 tests.

For now we have supported the riscv big endian targets, so
xfail riscv64*-*-* for ld-scripts/empty-address-2 tests, to
cover both little endian and big endian targets.

ld/
    * testsuite/ld-scripts/empty-address-2a.d: xfail riscv64*-*-*.
    * testsuite/ld-scripts/empty-address-2b.d: Likewise.