aarch64: Fix up aarch64_simd_clone_compute_vecsize_and_simdlen [PR99542]

The gcc.dg/declare-simd.c test does not emit a warning with
-mabi=ilp32.

2021-03-16  Christophe Lyon  <christophe.lyon@linaro.org>

	PR target/99542
	gcc/testsuite/
	* gcc.dg/declare-simd.c (fn2): Expect a warning only under lp64.