[AArch64][SME] Add calling convention for __arm_get_current_vg (#93963)

Adds a calling convention for calls to the `__arm_get_current_vg`
support
routine, which preserves X1-X15, X19-X29, SP, Z0-Z31 & P0-P15.

See https://github.com/ARM-software/abi-aa/pull/263