Exclude spirv-tools from the all build target.

This CL moves spirv-tools so that it will only build what is requested.
This means that the tool binaries will not be built as part of tint as
they aren't needed.

Change-Id: Idd89b45facbe835948c57fe6dda6068657a2f822
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/29660
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Ryan Harrison <rharrison@chromium.org>

third_party/CMakeLists.txt

@@ -23,6 +23,6 @@ if(${TINT_BUILD_SPV_READER} OR ${TINT_BUILD_SPV_WRITER})
 
   if (NOT TARGET SPIRV-Tools)
     set(SPIRV_SKIP_TESTS ON CACHE BOOL ON)
-    add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/spirv-tools)
+    add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/spirv-tools EXCLUDE_FROM_ALL)
   endif()
 endif()