riscv: xtheadbb: Fix xtheadbb-li-rotr test for rv32

riscv: xtheadbb: Fix xtheadbb-li-rotr test for rv32

The test was introduced recently and tests a RV64-only feature.
However, when testing an RV32 compiler, the test gets executed as well
and fails with "cc1: error: ABI requires '-march=rv32'".
This patch fixes this by adding '-mabi=lp64' (like it is done for
other RV64-only tests as well).

Retested with RV32 and RV64 to ensure this won't pop up again.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadbb-li-rotr.c: Don't run for RV32.