This patch introduces a function to house the code needed to do the
DWARF64 detection dance. It also updates three instances of DWARF64
parsing code to use this function instead of the hand-rolled
This patch does _not_ attempt to handle the problem of detecting lengths
which extend past the size of the section, or cases when reads of a
single contribution accidentally escape beyond its specified length, but
I think it's useful in its own right. The idea is that this functionality will
be built on top of the function introduced here.