Fixed CMake builds for iOS (facebook#12744)

Summary:
This [ports the flags which were already defined in `build_tools`](https://github.com/facebook/rocksdb/blob/44aceb88d0de120847719c061aa3a8465daaee48/build_tools/build_detect_platform#L151) for CMake.

The flag `OS_MACOSX` in specific is necessary for proper endiness detection, [since they're required for proper inclusion of OSX endian header](https://github.com/facebook/rocksdb/blob/44aceb88d0de120847719c061aa3a8465daaee48/port/port_posix.h#L27).

Pull Request resolved: facebook#12744

Reviewed By: ltamasi

Differential Revision: D58317987

Pulled By: ajkr

fbshipit-source-id: 407e623ddb6afc9c48939d52f610281f59cf99af

CMakeLists.txt

@@ -488,6 +488,8 @@ if(CMAKE_SYSTEM_NAME MATCHES "Cygwin")
   add_definitions(-fno-builtin-memcmp -DCYGWIN)
 elseif(CMAKE_SYSTEM_NAME MATCHES "Darwin")
   add_definitions(-DOS_MACOSX)
+elseif(CMAKE_SYSTEM_NAME MATCHES "iOS")
+  add_definitions(-DOS_MACOSX -DIOS_CROSS_COMPILE)
 elseif(CMAKE_SYSTEM_NAME MATCHES "Linux")
   add_definitions(-DOS_LINUX)
 elseif(CMAKE_SYSTEM_NAME MATCHES "SunOS")