[sanitizer] Cache results of darwin_test_archs

[kubamracek]

Summary

For OS X builds of compiler-rt, we run darwin_test_archs to determine which architectures can the toolchain target. This detection takes quite a long time, and the result is always the same (as long as you don't upgrade your OS, system headers or toolchain). Let's cache the result.

[beanz]

LGTM.

compiler-rt/trunk/cmake/Modules/CompilerRTDarwinUtils.cmake

function(darwin_get_toolchain_supported_archs output_var)
   endif()
   
   set(${output_var} ${ARCHES} PARENT_SCOPE)
 endfunction()
 
 # This function takes an OS and a list of architectures and identifies the
 # subset of the architectures list that the installed toolchain can target.
 function(darwin_test_archs os valid_archs)
+  if(${valid_archs})
+    message(STATUS "Using cached valid architectures for ${os}.")
+    return()
+  endif()
+
   set(archs ${ARGN})
   message(STATUS "Finding valid architectures for ${os}...")
   set(SIMPLE_CPP ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/src.cpp)
   file(WRITE ${SIMPLE_CPP} "#include <iostream>\nint main() { std::cout << std::endl; return 0; }\n")
 
   set(os_linker_flags)
   foreach(flag ${DARWIN_${os}_LINKFLAGS})
     set(os_linker_flags "${os_linker_flags} ${flag}")
foreach(arch ${archs})
     try_compile(CAN_TARGET_${os}_${arch} ${CMAKE_BINARY_DIR} ${SIMPLE_CPP}
                 COMPILE_DEFINITIONS "-v -arch ${arch}" ${DARWIN_${os}_CFLAGS}
                 CMAKE_FLAGS "-DCMAKE_EXE_LINKER_FLAGS=${arch_linker_flags}"
                 OUTPUT_VARIABLE TEST_OUTPUT)
     if(${CAN_TARGET_${os}_${arch}})
       list(APPEND working_archs ${arch})
     endif()
   endforeach()
-  set(${valid_archs} ${working_archs} PARENT_SCOPE)
+  set(${valid_archs} ${working_archs}
+    CACHE STRING "List of valid architectures for platform ${os}.")
 endfunction()
 
 # This function checks the host cpusubtype to see if it is post-haswell. Haswell
 # and later machines can run x86_64h binaries. Haswell is cpusubtype 8.
 function(darwin_filter_host_archs input output)
   list_union(tmp_var DARWIN_osx_ARCHS ${input})
   execute_process(
     COMMAND sysctl hw.cpusubtype