[AArch64][SVE] Guard svbfloat16_t with feature macro in ACLE

Summary:
svbfloat16_t should only be defined if the __ARM_FEATURE_SVE_BF16
feature macro is enabled, similar to the scalar bfloat16_t type. Also,
arm_bf16.h should be included in arm_sve.h when
__ARM_FEATURE_BF16_SCALAR_ARITHMETIC is defined.

Patch also contains a fix for ld1ro intrinsic which should be guarded on
__ARM_FEATURE_SVE_BF16 rather than __ARM_FEATURE_BF16_SCALAR_ARITHMETIC,
and a fix for bfmmla test which was missing
__ARM_FEATURE_BF16_SCALAR_ARITHMETIC and -target-feature +bf16 in the
RUN line.

Reviewed By: fpetrogalli

Differential Revision: https://reviews.llvm.org/D82178