Workaround for an issue with Cmake builds that happens when cross-com…

…piling using Cmake on macOS x86_64 CPUs with target CPU arm64 (facebook#12240)

Summary:
This is a temporary workaround for Cmake bug that only sets the correct CMAKE_SYSTEM_PROCESSOR for cross-compilation when target CMAKE_SYSTEM_NAME differs from CMAKE_HOST_NAME:
https://gitlab.kitware.com/cmake/cmake/-/issues/25640

Fix cross-compilation on macOS x86_64 CPUs with target CPU arm64 by manually setting CMAKE_SYSTEM_PROCESSOR to the cross-compilation target whenever CMAKE_SYSTEM_PROCESSOR doesn't match CMAKE_OSX_ARCHITECTURES after project() call. This is probably a Cmake bug that happens on macOS.
Closes facebook#12239

The issue happens when RocksDB is built using the follwoing command:
cmake -G "Unix Makefiles" -DCMAKE_SYSTEM_PROCESSOR=arm64 ..
The build itself succeeds, but because Cmake wrongly sets CMAKE_SYSTEM_PROCESSOR to x86_64 instead of arm64 and causes crc32c_arm64.cc not to be compiled.
This in turn makes the project fails any linking with RocksDB:

```
Undefined symbols for architecture arm64:
"crc32c_arm64(unsigned int, unsigned char const*, unsigned long)", referenced from:
rocksdb::crc32c::ExtendARMImpl(unsigned int, char const*, unsigned long) in librocksdb.a(crc32c.cc.o)
```

Pull Request resolved: facebook#12240

Reviewed By: cbi42

Differential Revision: D54811365

Pulled By: pdillinger

fbshipit-source-id: 0958e3092806dadd2f61d582b7251af13a5f3f06

CMakeLists.txt

@@ -44,6 +44,29 @@ project(rocksdb
   HOMEPAGE_URL https://rocksdb.org/
   LANGUAGES CXX C ASM)
 
+if(APPLE)
+  # On macOS Cmake, when cross-compiling, sometimes CMAKE_SYSTEM_PROCESSOR wrongfully stays
+  # the same as CMAKE_HOST_SYSTEM_PROCESSOR regardless the target CPU.
+  # The manual call to  set(CMAKE_SYSTEM_PROCESSOR) has to be set after the project() call.
+  # because project() might reset CMAKE_SYSTEM_PROCESSOR back to the value of CMAKE_HOST_SYSTEM_PROCESSOR.
+  # Check if CMAKE_SYSTEM_PROCESSOR is not equal to CMAKE_OSX_ARCHITECTURES
+  if(NOT CMAKE_OSX_ARCHITECTURES STREQUAL "")
+    if(NOT CMAKE_SYSTEM_PROCESSOR STREQUAL CMAKE_OSX_ARCHITECTURES)
+      # Split CMAKE_OSX_ARCHITECTURES into a list
+      string(REPLACE ";" " " ARCH_LIST ${CMAKE_OSX_ARCHITECTURES})
+      separate_arguments(ARCH_LIST UNIX_COMMAND ${ARCH_LIST})
+      # Count the number of architectures
+      list(LENGTH ARCH_LIST ARCH_COUNT)
+      # Ensure that exactly one architecture is specified
+      if(NOT ARCH_COUNT EQUAL 1)
+          message(FATAL_ERROR "CMAKE_OSX_ARCHITECTURES must have exactly one value. Current value: ${CMAKE_OSX_ARCHITECTURES}")
+      endif()
+      set(CMAKE_SYSTEM_PROCESSOR ${CMAKE_OSX_ARCHITECTURES})
+      message(STATUS "CMAKE_SYSTEM_PROCESSOR is manually set to ${CMAKE_SYSTEM_PROCESSOR}")
+    endif()
+  endif()
+endif()
+
 if(POLICY CMP0042)
   cmake_policy(SET CMP0042 NEW)
 endif()