Use portable modifiers/attributes from llvm/Support/Compiler.h

[Simon Moll]

RV's implementation uses GCC/Linux-specific features that will not compile on other systems (eg __attribute__((noreturn)) with MSVC, see #13).

The LLVM header llvm/Support/Compiler.h provides portable modifiers/attributes that work on all compilers that LLVM supports. We should use those macros were possible.