riscv: selftests: Fix vsetivli args for clang

Clang does not support implicit LMUL in the vset* instruction sequences.
Introduce an explicit LMUL in the vsetivli instruction.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Fixes: 9d5328ee

 ("riscv: selftests: Add signal handling vector tests")
Link: https://lore.kernel.org/r/20240702-fix_sigreturn_test-v1-1-485f88a80612@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>