python: hide symbols except for PyInit on macOS (#1506)

Description: This PR modifies the build recipe for `envoy_engine.so` to hide all symbols except for `_PyInit_envoy_engine` by default on maOS, so that envoy_engine can be loaded alongside a [protobuf](http://pypi.org/project/protobuf) wheel without breaking.

Loading in protobuf alongside `envoy_engine.so` would register two sets of the same symbols and, on macOS, the runtime linker chooses the first symbol it finds causing problems. See the [similar protobuf change](protocolbuffers/protobuf#8346) and its [sister change in grpc](grpc/grpc#24992) for more information on what's going on.

Risk Level: Low
Testing: See the now-closed #1504 for how I've been testing this.
Docs Changes: N/A
Release Notes: N/A

Signed-off-by: Cerek Hillen <chillen@lyft.com>
Signed-off-by: JP Simard <jp@jpsim.com>

mobile/library/python/BUILD

@@ -31,6 +31,10 @@ pybind_extension(
     srcs = [
         "module_definition.cc",
     ],
+    linkopts = select({
+        "//bazel:darwin": ["-Wl,-exported_symbol,_PyInit_envoy_engine"],
+        "//conditions:default": [],
+    }),
     visibility = ["//visibility:public"],
     deps = [
         ":envoy_engine_lib",