Fix SpecCPU2017's dependency from timeit-target to build-timeit-target

Building SpecCPU2017 using cmake raises this error:

```
CMake Error at External/SPEC/SpecCPU2017.cmake:263 (add_dependencies):
  The dependency target "timeit-target" of target "imagevalidate_525-target"
  does not exist.
Call Stack (most recent call first):
  External/SPEC/CINT2017rate/525.x264_r/CMakeLists.txt:24 (speccpu2017_validate_image)
```

I think `timeit-target` should be `build-timeit-target` instead because cmake's progress output has this line:

```
[  3%] Built target build-timeit-target
```

Fixing this resolves the dependency error.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D100734
diff --git a/External/SPEC/SpecCPU2017.cmake b/External/SPEC/SpecCPU2017.cmake
index 6bae1df..327055c 100644
--- a/External/SPEC/SpecCPU2017.cmake
+++ b/External/SPEC/SpecCPU2017.cmake
@@ -272,7 +272,7 @@
     add_executable(${VALIDATOR} ${_validator_sources})
     target_link_libraries(${VALIDATOR} m)
     set_target_properties(${VALIDATOR} PROPERTIES COMPILE_FLAGS "-DSPEC")
-    add_dependencies(${VALIDATOR} timeit-target)
+    add_dependencies(${VALIDATOR} build-timeit-target)
   endif ()
 
   if ((NOT DEFINED _carg_SUITE_TYPE) OR (${BENCHMARK_SUITE_TYPE} IN_LIST _carg_SUITE_TYPE))