Allow --build_wasm on a mac system (microsoft#16761)

### Description
Changes allow downloading prebuilt protoc compiler when building
WebAssebly version on mac systems.
Otherwise it tries to build a js/wasm version of protoc and throws an
error while executing it: "protoc.js permission denied"


### Motivation and Context
I need to switch between my main working computer and a PC to make
changes to WebAssebly build. Would like not to do that anymore.

cmake/deps.txt

@@ -32,6 +32,7 @@ protoc_win32;https://github.com/protocolbuffers/protobuf/releases/download/v21.1
 protoc_linux_x64;https://github.com/protocolbuffers/protobuf/releases/download/v21.12/protoc-21.12-linux-x86_64.zip;338462004aa5be9fba45b35b5b4be43f69b47a90
 protoc_linux_x86;https://github.com/protocolbuffers/protobuf/releases/download/v21.12/protoc-21.12-linux-x86_32.zip;61fdbe7d6360e065ec6fea23bca2cca673115fb8
 protoc_linux_aarch64;https://github.com/protocolbuffers/protobuf/releases/download/v21.12/protoc-21.12-linux-aarch_64.zip;df9d45470b0b8cf939dd2f0ec6b88e9cafc4d617
+protoc_mac_universal;https://github.com/protocolbuffers/protobuf/releases/download/v21.12/protoc-21.12-osx-universal_binary.zip;23710c3d1c2036d8d65a6a22234372fa2d7af9ef
 psimd;https://github.com/Maratyszcza/psimd/archive/072586a71b55b7f8c584153d223e95687148a900.zip;1f5454b01f06f9656b77e4a5e2e31d7422487013
 pthreadpool;https://github.com/Maratyszcza/pthreadpool/archive/1787867f6183f056420e532eec640cba25efafea.zip;e43e80781560c5ab404a4da20f34d846f5f5d101
 pybind11;https://github.com/pybind/pybind11/archive/refs/tags/v2.10.1.zip;769b6aa67a77f17a770960f604b727645b6f6a13

cmake/external/onnxruntime_external_deps.cmake

@@ -107,7 +107,7 @@ FetchContent_Declare(
 )
 
 # Download a protoc binary from Internet if needed
-if(CMAKE_CROSSCOMPILING AND NOT ONNX_CUSTOM_PROTOC_EXECUTABLE AND NOT CMAKE_OSX_ARCHITECTURES)
+if(CMAKE_CROSSCOMPILING AND NOT ONNX_CUSTOM_PROTOC_EXECUTABLE)
   # This part of code is only for users' convenience. The code couldn't handle all cases. Users always can manually
   # download protoc from Protobuf's Github release page and pass the local path to the ONNX_CUSTOM_PROTOC_EXECUTABLE
   # variable.
@@ -141,6 +141,14 @@ if(CMAKE_CROSSCOMPILING AND NOT ONNX_CUSTOM_PROTOC_EXECUTABLE AND NOT CMAKE_OSX_
       set(ONNX_CUSTOM_PROTOC_EXECUTABLE ${protoc_binary_SOURCE_DIR}/bin/protoc)
 	  set(PROTOC_EXECUTABLE ${ONNX_CUSTOM_PROTOC_EXECUTABLE})
     endif()
+  elseif (CMAKE_SYSTEM_NAME STREQUAL "Emscripten" AND CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
+    FetchContent_Declare(protoc_binary URL ${DEP_URL_protoc_mac_universal} URL_HASH SHA1=${DEP_SHA1_protoc_mac_universal})
+    FetchContent_Populate(protoc_binary)
+    if(protoc_binary_SOURCE_DIR)
+      message("Use prebuilt protoc")
+      set(ONNX_CUSTOM_PROTOC_EXECUTABLE ${protoc_binary_SOURCE_DIR}/bin/protoc)
+      set(PROTOC_EXECUTABLE ${ONNX_CUSTOM_PROTOC_EXECUTABLE})
+    endif()
   endif()
 endif()