Specify SDK version explicitly when linking for iOS

If `-sdk_version` option is absent, ld infers the version from
`-syslibroot` path.

Since sysroot directory name in our dependencies differs from the
Xcode SDK names, the resulting binary didn't contain correct SDK version
specification, and this was leading to device-orientation-related bugs
on iOS 11.

backend.native/konan.properties

@@ -64,7 +64,7 @@ llvmLtoFlags.ios =
 llvmLtoOptFlags.ios = -O3 -function-sections
 linkerDebugFlags.ios = -S
 llvmLtoNooptFlags.ios = -O1
-linkerKonanFlags.ios = -lc++ -lobjc
+linkerKonanFlags.ios = -lc++ -lobjc -sdk_version 10.2
 linkerOptimizationFlags.ios = -dead_strip
 osVersionMinFlagLd.ios = -iphoneos_version_min
 osVersionMinFlagClang.ios = -miphoneos-version-min
@@ -84,7 +84,7 @@ libffiDir.ios_sim = libffi-3.2.1-2-darwin-ios-sim
 llvmLtoFlags.ios_sim =
 llvmLtoOptFlags.ios_sim = -O3 -function-sections
 llvmLtoNooptFlags.ios_sim = -O1
-linkerKonanFlags.ios_sim = -lc++ -lobjc
+linkerKonanFlags.ios_sim = -lc++ -lobjc -sdk_version 10.2
 linkerOptimizationFlags.ios_sim = -dead_strip
 linkerDebugFlags.ios_sim = -S
 osVersionMinFlagLd.ios_sim = -ios_simulator_version_min