Support custom expedited register set in gdb-remote

This patch adds capability to introduce a custom expedited register set
in gdb remote. Currently we send register set 0 as expedited register set
but for the case of AArch64 SVE we intend to send additional information
about SVE registers size/offset configuration which can be calculated
from vg register. Therefore we will expedited Vg register in case of
AArch64 is in SVE mode to speedup register configuration calculations.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D82853