Recommend using Xcode SDK directly on Linux by extracting it instead of using OSXCross

[mccakit]

Just using llvm is enough to cross compile to mac and ios from a linux host, osxcross is not needed at all and is mostly counterproductive. What needs to be done is:

• tellthe users to get an Apple account, or borrow one
• download XCode
• Extract it using https://github.com/bitcoin-core/apple-sdk-tools
• Use the sysroot with clang with fllags like -isysroot and -mmacos-version-min and link to the compiler-rt libclang_rt.osx.a

Osxcross is just not worth dealing with, when you can just use llvm