[core] mason geojsonvt-2.1.6.3

- [osx] set -fvisibility-inlines-hidden in gyp/common.gypi to silence
  mismatched visibility linker warnings
- [linux] set cxx_host in GYP_FLAGS to set -fabi-version=0 on gcc builds
  to use clang-built mason binaries
- [ios] update symbol visibility for iOS tests
- [ios] link libgeojsonvt.a in iOS tests xcodeproj
  - add libgeojsonvt.a to General -> Linked Frameworks and Libraries
  - add mason_packages (recursive) to Build Settings -> Library Search Path
- [ios] add libuv and geojsonvt first in iOS libtool smush to ensure symbols
  are found by later compilation units where they are undefined

.gitmodules

@@ -10,6 +10,3 @@
 	path = platform/ios/vendor/SMCalloutView
 	url = https://github.com/nfarina/calloutview.git
 
-[submodule "src/mbgl/util/geojsonvt"]
-	path = src/mbgl/util/geojsonvt
-	url = https://github.com/mapbox/geojson-vt-cpp