[AArch64][SME2] Add ZT0 attributes to SMEAttrs (#77607)

This patch extends SMEAttrs to interpret the following new attributes,
which are mutually exclusive and apply to SME2 only:
  - aarch64_sme_zt0_in (ZT0_In)
  - aarch64_sme_zt0_out (ZT0_Out)
  - aarch64_sme_zt0_inout (ZT0_InOut)
  - aarch64_sme_zt0_new (ZT0_New)
  - aarch64_sme_zt0_preserved (ZT0_Preserved)

ZT0_In, ZT0_Out, ZT0_InOut & ZT0_Preserved are all considered to share
ZT0. These attributes will be required by later patches to determine
if ZT0 should be preserved around function calls, or cleared on entry
to the function.