[runtimes] Do not set XXX_STANDALONE_BUILD for libc++/abi/unwind

[ldionne]

Summary

The runtimes build was lying to the various runtimes builds by setting
XXX_STANDALONE_BUILD=ON when they are really not being built standalone.
Only COMPILER_RT_STANDALONE_BUILD appears to be necessary, but setting it
for the other runtimes actually breaks everything.

[phosek]

LGTM

llvm/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
 
   foreach(entry ${runtimes})
     get_filename_component(projName ${entry} NAME)
 
     # TODO: Clean this up as part of an interface standardization
     string(REPLACE "-" "_" canon_name ${projName})
     string(TOUPPER ${canon_name} canon_name)
 
-    # The subdirectories need to treat this as standalone builds. D57992 tried
-    # to get rid of this, but the runtimes treat *_STANDALONE_BUILD=OFF as if
-    # llvm & clang are configured in the same CMake, and setup dependencies
+    # compiler-rt needs to treat this as standalone builds. D57992 tried
+    # to get rid of it for compiler-rt, but it treats *_STANDALONE_BUILD=OFF
+    # as if llvm & clang are configured in the same CMake, and setup dependencies
     # against their targets.
+    if ("${canon_name}" STREQUAL "COMPILER_RT")
       set(${canon_name}_STANDALONE_BUILD ON)
+    endif()
 
     if(LLVM_RUNTIMES_LIBDIR_SUBDIR)
       set(${canon_name}_LIBDIR_SUBDIR "${LLVM_RUNTIMES_LIBDIR_SUBDIR}" CACHE STRING "" FORCE)
     endif()
 
     # Setting a variable to let sub-projects detect which other projects
     # will be included under here.
     set(HAVE_${canon_name} ON)