Changed GccVersionDeterminer so that it can extract the default target architecture for gcc/clang.
Moved targetPlatform up to BinaryToolSpec and use this to decide whether to pass -fPIC to the GCC compiler or not.`
GCC can build x64 binaries on Windows in general, so remove this check. Still need to improve detection of whether what we find in the path can build x86 or x64 binaries or both or neither, but this is true of Linux and OS X too.
Some notes about native binary improvements.
Some updates to the native binaries chapter:

- Listed a few 'unofficially supported' tool chain + platform combinations.
- Added some details about what you need to install in order to build on various platforms.
Listed a few more things we should add to the CI build
Minor spec update