Extend PEI to emit a DWARF expression for StackOffsets that have
a fixed and scalable component. This means the expression that needs
to be added is either:
<base> + offset
<base> + offset + scalable_offset * scalereg
where for SVE, the scale reg is the Vector Granule Dwarf register, which
encodes the number of 64bit 'granules' in an SVE vector and which
the debugger can evaluate at runtime.