Use correct sign in get_mpz

This changes dwarf2/read.c:get_mpz to use the correct sign-extension
function.  Normally a rational constant uses signed values, but a
purely unsigned form also seems fine here.  This adds a new
attribute::form_is_strictly_unsigned, which is more precise than
form_is_unsigned (which accepts a lot of forms that aren't really for
ordinary constants).

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