Make xctool play nice with Xcode 5

The way we trick Xcode into building iOS dylibs has to change.  We used to be able to trick it just by setting OTHER_CFLAGS and OTHER_LDFLAGS, but something changed in their linker.  Not sure what - they haven't open-sourced this version yet.  As a workaround for now, we'll set our own LD that strips out macosx-specific arguments.  (Works for both v4 and v5)

xcodebuild-shim and xcodebuild-fastsettings-shim have to change, too.  Some class and method names are different in v5.  I've verified these changes work for v4 and v5.

I am noticing that building for `iphonesimulator` is not working for Xcode5, but it's not working in plain `xcodebuild` either so I'm assuming it's a bug (hopefully they'll fix!).  Building for iphoneos is fine, though.  The error you get is... 
```
warning: no rule to process file '.../blah.m' of type sourcecode.c.objc for architecture i386
```

Note - this doesn't include support for XCTest yet.  Will do separately.