Do not attempt to use _objc_msgForward_stret on 64-bit architectures.

It does not exist, and is no longer necessary:
https://twitter.com/gparker/status/378079715824660480 and replies
("struct return address is passed in a non-parameter register").
Fix format string warnings.
Fix "using `stringWithString:` with a literal is redundant" warnings.
Disable "undeclared selector warnings" in the iOS unit tests.

The tests intentionally use undeclared selectors in several places.
Merge pull request #16 from inkling/jeff/Xcode_5_1

Xcode 5.1 compatibility.