target/arm: Restructure has_vfp_d32 test

target/arm: Restructure has_vfp_d32 test

One cannot test for feature aa32_simd_r32 without first
testing if AArch32 mode is supported at all.  This leads to

qemu-system-aarch64: ARM CPUs must have both VFP-D32 and Neon or neither

for Apple M1 cpus.

We already have a check for ARMv8-A never setting vfp-d32 true,
so restructure the code so that AArch64 avoids the test entirely.

Reported-by: Mads Ynddal <mads@ynddal.dk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Mads Ynddal <m.ynddal@samsung.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Mads Ynddal <m.ynddal@samsung.com>
Message-id: 20230619140216.402530-1-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>