Update on "[executorch][docs] runtime-overview.md"

First draft of a high-level document describing the runtime.

Differential Revision: [D49852127](https://our.internmc.facebook.com/intern/diff/D49852127/)

[ghstack-poisoned]

Author: dbort

CMakeLists.txt

@@ -162,7 +162,7 @@ include(${EXECUTORCH_SRCS_FILE})
 # _default_build_host_targets: The default value for the
 # EXECUTORCH_BUILD_HOST_TARGETS option.
 set(_default_build_host_targets ON)
-if(CMAKE_TOOLCHAIN_FILE MATCHES ".*ios\.toolchain\.cmake$")
+if(CMAKE_TOOLCHAIN_FILE MATCHES ".*iOS\.cmake$")
   # TODO(dbort): Refactor toolchain-specific matching. Ideally this file would
   # not know about specific targets, and work with more abstract concepts like
   # "is cross compiling".

backends/xnnpack/CMakeLists.txt

@@ -91,7 +91,7 @@ target_link_options_shared_lib(xnnpack_backend)
 list(APPEND xnn_executor_runner_libs xnnpack_backend)
 
 # ios can only build library but not binary
-if(NOT CMAKE_TOOLCHAIN_FILE MATCHES ".*ios\.toolchain\.cmake$")
+if(NOT CMAKE_TOOLCHAIN_FILE MATCHES ".*iOS\.cmake$")
   #
   # xnn_executor_runner: Like executor_runner but with XNNPACK, the binary will
   # be at ${CMAKE_BINARY_DIR}/backends/xnnpack