[AArch64][SME] Save VG for unwind info when changing streaming-mode (#83301)

If a function requires any streaming-mode change, the vector granule
value must be stored to the stack and unwind info must also describe the
save of VG to this location.

This patch adds VG to the list of callee-saved registers and increases
the
callee-saved stack size if the function requires streaming-mode changes.
A new type is added to RegPairInfo, which is also used to skip restoring
the register used to spill the VG value in the epilogue.

See
https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst