[CMake][libc] Distinguish compiler and run failures

[phosek]

Summary

This is useful for debugging issues, for example when cross-compiling.

[gchatelet]

Thx!

libc/cmake/modules/LLVMLibCCheckCpuFeatures.cmake

compute_flags(
     REQUIRE ${CHECK_DEFINED_REQUIRE}
     REJECT  ${CHECK_DEFINED_REJECT})
   try_run(
     run_result compile_result "${CMAKE_CURRENT_BINARY_DIR}/check_${feature}"
     "${CMAKE_CURRENT_BINARY_DIR}/cpu_features/check_cpu_features.cpp"
     COMPILE_DEFINITIONS ${flags}
     COMPILE_OUTPUT_VARIABLE compile_output
     RUN_OUTPUT_VARIABLE run_output)
-  if(${compile_result} AND ("${run_result}" EQUAL 0))
+  if("${run_result}" EQUAL 0)
     set(${output_var}
         "${run_output}"
         PARENT_SCOPE)
+  elseif(NOT ${compile_result})
+    message(FATAL_ERROR "Failed to compile: ${compile_output}")
   else()
-    message(FATAL_ERROR "${compile_output}")
+    message(FATAL_ERROR "Failed to run: ${run_output}")
   endif()
 endfunction()
 
 # Populates the HOST_CPU_FEATURES list.
 # Use -march=native only when the compiler supports it.
 include(CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG("-march=native" COMPILER_SUPPORTS_MARCH_NATIVE)
 if(COMPILER_SUPPORTS_MARCH_NATIVE)
   _check_defined_cpu_feature(HOST_CPU_FEATURES MARCH native)
 else()
   _check_defined_cpu_feature(HOST_CPU_FEATURES)
 endif()