Introduce attribute::signed_constant

Introduce attribute::signed_constant

This introduces a new method, attribute::signed_constant.  This should
be used wherever DWARF specifies a signed integer constant, or where
this is implied by the context.  It properly handles sign-extension
for DW_FORM_data*.

To my surprise, there doesn't seem to be a pre-existing sign-extension
function.  I've added one to common-utils.h alongside the align
functions.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680