[AArch64][SME2] Enable bfm builtins for sme2 (#71927)

This patch enables the following builtins for SME2
    svbfmlslb_f32
    svbfmlslb_lane_f32
    svbfmlslt_f32
    svbfmlslt_lane_f32

Patch by: Kerry McLaughlin <kerry.mclaughlin@arm.com>

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Co-authored-by: Matthew Devereau <matthew.devereau@arm.com>