Forcibly disable use of libstdc++ C++11 ABI on Mono LLVM (#32234)

Forcibly disable use of libstdc++ C++11 ABI on Mono LLVM (#32234)

* Forcibly disable use of libstdc++ C++11 ABI on Mono LLVM

This feature is documented at https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html

Basically, the C++ ABI of the LLVM libraries needs to match the ABI of Mono.
The CentOS7 images we build with on CI use the old (GCC 4 style) ABI by
default, which ends up embedded in our LLVM nupkgs. This results in linker
errors when building runtime.git on distributions which use the C++11 ABI
instead, like Ubuntu 16.04+:

```
./.libs/libmini.a(llvm-jit.o): In function `mono_llvm_create_ee':
/home/directhex/Projects/runtime/src/mono/mono/mini/llvm-jit.cpp:447: undefined reference to `MonoEHFrameSymbol[abi:cxx11]'
./.libs/libmini.a(llvm-jit.o): In function `llvm::RTDyldMemoryManager::getSymbolAddress(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/home/directhex/Projects/runtime/artifacts/obj/mono/Linux.x64.Debug/llvm/include/llvm/ExecutionEngine/RTDyldMemoryManager.h:84: undefined reference to `llvm::RTDyldMemoryManager::getSymbolAddressInProcess(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
./.libs/libmini.a(llvm-jit.o):(.data.rel.ro+0x90): undefined reference to `llvm::RTDyldMemoryManager::getPointerToNamedFunction(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)'
```

Adding a define to CXXFLAGS, -D _GLIBCXX_USE_CXX11_ABI=0 forces the build
to use the same ABI as the LLVM nupkgs we produce.

The alternative would be to modify our LLVM build to enable the
non-default C++11 ABI.

* Also add flag in Release configuration