Add LLVM.init_native_target and LLVM.init_all_targets

[HertzDevil]

LLVM's targets need to be initialized before they can be used. The Crystal compiler does this in Crystal::Codegen::Target#to_target_machine, but for external code that uses the LLVM wrappers directly, there is no simple way to do this. This PR adds two convenience methods which are more or less equivalent to LLVM's LLVMInitializeNativeTarget and LLVMInitializeAllTargets, both static line functions in llvm-c/Target.h, except the latter only initializes targets already supported by Crystal itself (e.g. no RISC-V or PowerPC at the moment even if llvm-config indicates their availability).