support exception-handling feature in gcc

[arvid@…]

It's not clear to me if the exception-handling is deliberately excluded from the gcc toolset or not, given how small this change is, it almost seems like it. Anyway, I can't see a reason not to support this feature for GCC. Appending this at the end of build/v2/tools/gcc.jam makes it work for me (on darwin).

toolset.flags gcc OPTIONS <exception-handling>off : -fno-exceptions ;
toolset.flags gcc OPTIONS <exception-handling>on : -fexceptions ;