When enabling C++17, LLVM uses std::shared_mutex instead of its own
implementation. On macOS it requires version 10.12 (Sierra), while
Chrome must still support 10.11. However, we don't need LLVM as Reactor
backend, except for ARM64, but Mac devices with such CPU have a more
recent OS version.

llvm-10.0/llvm/include/llvm/Support/RWMutex.h:100:8: error: 'shared_mutex' is unavailable: introduced in macOS 10.12

Bug: b/174843857
Change-Id: Ie24095aa17d22072cfb9d655b87252cbea5209d0
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52030Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>