libdw: Use signedness of subrange type to determine array bounds

libdw: Use signedness of subrange type to determine array bounds

When calculating the array size check if the subrange has an associate
type, if it does then check the type to determine whether the upper
and lower values need to be interpreted as signed of unsigned
values. We default to signed because that is what the testcase
run-aggregate-size.sh testfile-size4 expects (this is an hardwritten
testcase, we could have chosen a different default).

https://sourceware.org/bugzilla/show_bug.cgi?id=28294

Signed-off-by: Mark Wielaard <mark@klomp.org>