meson: llvm: Use llvm-config's --cxxflags when building llvmjit

meson: llvm: Use llvm-config's --cxxflags when building llvmjit

Otherwise we don't use LLVM's flags when building llvmjit_wrap.cpp and
llvmjit_inline.cpp. That can cause compile time failures if the C++ compiler
doesn't default to a new enough C++ standards version and link time failures
due to ABI influencing flags like -fno-rtti.