[CMake][runtimes] Process common options in runtimes build

[phosek]

Summary

This was removed in D39932 but turned out this is actually needed
because runtimes such as compiler-rt and libc++ rely on common options
processing for setting certain flags such as -ffunction-sections and
-fdata-sections.

llvm/trunk/runtimes/CMakeLists.txt

if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
   check_c_compiler_flag(-nodefaultlibs LLVM_HAS_NODEFAULTLIBS_FLAG)
   if(LLVM_HAS_NODEFAULTLIBS_FLAG)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")
     if(LLVM_HAS_C_LIB)
       list(APPEND CMAKE_REQUIRED_LIBRARIES c)
     endif()
   endif()
 
+  # Avoid checking whether the compiler is working.
+  set(LLVM_COMPILER_CHECKED ON)
+
+  # Handle common options used by all runtimes.
+  include(AddLLVM)
+  include(HandleLLVMOptions)
+
   set(CMAKE_REQUIRED_FLAGS ${SAFE_CMAKE_REQUIRED_FLAGS})
   set(CMAKE_REQUIRED_LIBRARIES ${SAFE_CMAKE_REQUIRED_LIBRARIES})
 
   if(NOT LLVM_RUNTIMES_PREFIX)
     set(LLVM_RUNTIMES_PREFIX "${LLVM_RUNTIMES_TARGET}/")
   endif()
 
   foreach(entry ${runtimes})