[lldb][AArch64] Add SME streaming vector length pseduo register

This adds a register "svg" which mirrors SVE's "vg" register.
This reports the streaming vector length at all times, read
from the ZA ptrace header.

This register is needed first to implement ZA resizing as
the streaming vector length changes. Like vg, svg will be
expedited to the client so it can reconfigure its register
definitions.

The other use is for users to be able to know the streaming
vector length without resorting to counting the (many, many)
bytes in ZA, or temporarily entering streaming mode (which
would be destructive).

Some refactoring has been done so we don't have to recalculate the
register offsets twice.

Testing for this will come in a later patch.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D159503