builtins: re-enable building assembly sources

Re-apply SVN r213684 which was reverted in SVN r213724 since it broke the
build bots.  Add a tweak to enable inclusion of the assembly sources in
standalone build as well.

Original commit message:

This patch address the PR20360. The CMake assembler build system
ignores the .S assembly files in builtins library build. This patch
fixes the issue.

llvm-svn: 214013

compiler-rt/CMakeLists.txt

@@ -9,7 +9,7 @@
 
 # Check if compiler-rt is built as a standalone project.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
-  project(CompilerRT)
+  project(CompilerRT ASM C CXX)
   set(COMPILER_RT_STANDALONE_BUILD TRUE)
 else()
   set(COMPILER_RT_STANDALONE_BUILD FALSE)

compiler-rt/lib/builtins/CMakeLists.txt

@@ -251,6 +251,7 @@ add_custom_target(builtins)
 if (NOT WIN32)
   foreach(arch x86_64 i386 arm)
     if(CAN_TARGET_${arch})
+      set_source_files_properties(${${arch}_SOURCES} PROPERTIES LANGUAGE C)
       add_compiler_rt_runtime(clang_rt.builtins-${arch} ${arch} STATIC
         SOURCES ${${arch}_SOURCES}
         CFLAGS "-std=c99")