[CMake][runtimes] Don't process common options in runtimes build

[phosek]

Summary

This is no longer needed for any of the runtimes build and it breaks
in case we don't have the working compiler yet, e.g. when building
a compiler that uses compiler-rt and libc++ as a default runtime,
because these common options check whether these are available.

[beanz]

LGTM.

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()
 
-  # 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})