aarch64: Extend aarch64-autovec-preference==2 to 128-bit SVE

When compiling with -msve-vector-bits=128, aarch64_preferred_simd_mode
would pass the same vector width to aarch64_simd_container_mode for
both SVE and Advanced SIMD, and so Advanced SIMD would always “win”.
This patch instead makes it choose directly between SVE and Advanced
SIMD modes, so that aarch64-autovec-preference==2 and
aarch64-autovec-preference==4 work for this configuration.

(aarch64-autovec-preference shouldn't affect aarch64_simd_container_mode
because that would have an ABI impact for things like GNU vectors.)

gcc/
	* config/aarch64/aarch64.c (aarch64_preferred_simd_mode): Use
	aarch64_full_sve_mode and aarch64_vq_mode directly, instead of
	going via aarch64_simd_container_mode.