dwflpp/prologue: use function's decl_line for older GCCs

dwflpp/prologue: use function's decl_line for older GCCs

This patch restores a heuristic we previously used, which worked well in
previous versions of GCC. In a nutshell, decl_line is now meaningless
for determining prologue ends, but for versions of GCC prior to 4.4.0,
it can be useful since it represents the lineno of '{', which should be
the same lineno as the entrypc, otherwise we consider it naked.