Fix strict-aliasing warnings from unwind-dw2-fde.c

[Roland McGrath]

> That's the intention of my patch - use memcpy, in an inline function so 
> the compiler can see everything and optimize to the same code as for type 
> punning as appropriate.

Please verify that the change does not change the compiled code for x86-64.
If it doesn't change the code, or any changes don't reduce performance,
then this is OK.