[compiler-rt] Don't build llvm-lit in RUNTIMES-BUILD

With runtimes-build, we lost map config for llvm-lit, so we can NOT run
lit from source dir anymore.
All the subdir target like: ninja check-llvm-codegen-powerpc will fail
too.

We will get some cfg error like:
  File "../lvm-project/llvm/test/lit.cfg.py", line 21, in <module>
    config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)
AttributeError: 'NoneType' object has no attribute 'use_lit_shell'

This is reset of map config in llvm-lit rebuild.
We already have llvm-lit in runtimes-build, so don't build it.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D87002

compiler-rt/CMakeLists.txt

@@ -544,7 +544,8 @@ add_subdirectory(lib)
 if(COMPILER_RT_INCLUDE_TESTS)
   add_subdirectory(unittests)
   add_subdirectory(test)
-  if (COMPILER_RT_STANDALONE_BUILD)
+  # Don't build llvm-lit for runtimes-build, it will clean up map_config.
+  if (COMPILER_RT_STANDALONE_BUILD AND NOT RUNTIMES_BUILD)
     # If we have a valid source tree, generate llvm-lit into the bin directory.
     # The user can still choose to have the check targets *use* a different lit
     # by specifying -DLLVM_EXTERNAL_LIT, but we generate it regardless.