Add AllTargetsBindings sublibrary instead of having static inlines in the llvm-c headers.

This new library will be linked in when using the "all-targets"
component and contains the LLVMInitializeAll* functions.

This means that those functions will exist as real symbols in
the shared library, and can therefore can be called from
bindings that are using ffi the shared library.

llvm-svn: 192690