[vcpkg] Fix cross compiling macOS (#26240)

This follows the pattern in the Windows toolchain with respect to
setting VCPKG_CMAKE_SYSTEM_VERSION and also how arm64 macOS can execute
x86_64 through Rosetta.

scripts/toolchains/osx.cmake

@@ -1,11 +1,6 @@
 if(NOT _VCPKG_OSX_TOOLCHAIN)
 set(_VCPKG_OSX_TOOLCHAIN 1)
-if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
-    set(CMAKE_CROSSCOMPILING OFF CACHE BOOL "")
-    set(CMAKE_SYSTEM_VERSION "${CMAKE_HOST_SYSTEM_VERSION}" CACHE STRING "")
-else()
-    set(CMAKE_SYSTEM_VERSION "17.0.0" CACHE STRING "")
-endif()
+
 set(CMAKE_SYSTEM_NAME Darwin CACHE STRING "")
 
 set(CMAKE_MACOSX_RPATH ON CACHE BOOL "")
@@ -22,6 +17,23 @@ if(NOT DEFINED CMAKE_SYSTEM_PROCESSOR)
     endif()
 endif()
 
+if(DEFINED VCPKG_CMAKE_SYSTEM_VERSION)
+    set(CMAKE_SYSTEM_VERSION "${VCPKG_CMAKE_SYSTEM_VERSION}" CACHE STRING "" FORCE)
+endif()
+
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
+    if(CMAKE_SYSTEM_PROCESSOR STREQUAL CMAKE_HOST_SYSTEM_PROCESSOR)
+        set(CMAKE_CROSSCOMPILING OFF CACHE STRING "")
+    elseif(CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "ARM64")
+        # arm64 macOS can run x64 binaries
+        set(CMAKE_CROSSCOMPILING OFF CACHE STRING "")
+    endif()
+
+    if(NOT DEFINED CMAKE_SYSTEM_VERSION)
+        set(CMAKE_SYSTEM_VERSION "${CMAKE_HOST_SYSTEM_VERSION}" CACHE STRING "")
+    endif()
+endif()
+
 get_property( _CMAKE_IN_TRY_COMPILE GLOBAL PROPERTY IN_TRY_COMPILE )
 if(NOT _CMAKE_IN_TRY_COMPILE)
     string(APPEND CMAKE_C_FLAGS_INIT " -fPIC ${VCPKG_C_FLAGS} ")