Describe changes to VG in aadwarf64

[rsandifo-arm]

SME adds the concept of “streaming mode”, which can be entered using SMSTART and exited using SMSTOP. The SME ABI requires these changes in streaming mode to be naturally nested: a function entered in streaming mode must return in streaming mode, and a function entered in non-streaming mode must return in non-streaming mode.

The vector length in streaming mode can be different from the vector length in non-streaming mode. As a consequence, it's possible for the value of the DWARF VG pseudo register to change at an SMSTART and at an SMSTOP. Each such change in value could invalidate VG-based DWARF expressions for functions further up the call stack. For example, if a function F1 spills an SVE register to the stack, the size of its stack frame might depend on VG. If F1 then calls a function F2 that does an SMSTART, the SMSTART could change the value of VG. Trying to calculate the size of F1's stack frame using this new VG could give the wrong result.

This patch describes a mechanism for dealing with this. Although SME was the motivating use case, it's in principle possible for programs to change the SVE vector length independently of SME. We don't yet have a defined ABI for doing that, but the mechanism described here was always our back-up plan for handling this in DWARF, if it ever became necessary.