[basic-improvements] try/finally support for c/c++

[Aldy Hernandez]

> Please don't:
> 
> - Call this file 'unwind-c.c'.  That might lead people to think it had
>   something to do with stack unwinding, and it doesn't, it happens
>   after that.  Instead, call it something like 'eh-personality-c.c'.

That was rth's suggested name.  I'll name it whatever y'all want me to
name it.

> - Add this file in LIB2ADDEH.  It is needed on all targets, so
>   it should go in LIB2FUNCS_*.  (You'll understand this more when I post
>   the patch I just finished writing for EH support on Darwin.)

Ok.

Aldy