RISC-V: Specify -mabi in rv64 autovec testcase

On rv32 targets, this patch fixes:
FAIL: gcc.target/riscv/rvv/autovec/madd-split2-1.c -O3 -ftree-vectorize (test for excess errors)

cc1: error: ABI requires '-march=rv32'

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/madd-split2-1.c: Add -mabi=lp64d
	to dg-options.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>