[CMake] Pass -march=i686 when targeting i686 (PR24222)

[hans]

Summary

Clang will not define i686, even when the target triple is i686,
without -march=i686.

With this patch, the compiler-rt build will successfully detect that
Clang can target i686.

The open_memstream.cc test is a little funny. Before my patch, it
was invoked with "-m32 -m64". To make it work after my -march
change, I had to add '-march='''.

[samsonov]

LGTM. Thank you!

cmake/config-ix.cmake

183 ↗

(On Diff #30976)

Please mention PR24222 in a comment here.

test/sanitizer_common/TestCases/Linux/open_memstream.cc

2 ↗

(On Diff #30976)

Would -march=x86_64 work here?

[hans]

cmake/config-ix.cmake

183 ↗

(On Diff #30976)

Will do.

test/sanitizer_common/TestCases/Linux/open_memstream.cc

2 ↗

(On Diff #30976)

Yes, but it has to be spelled -march=x86-64. Would you prefer that?

[samsonov]

test/sanitizer_common/TestCases/Linux/open_memstream.cc

2 ↗

(On Diff #30976)

Yes please.

compiler-rt/trunk/cmake/config-ix.cmake

if(ANDROID)
   set(COMPILER_RT_OS_SUFFIX "-android")
 else()
   if("${LLVM_NATIVE_ARCH}" STREQUAL "X86")
     if(NOT MSVC)
       test_target_arch(x86_64 "" "-m64")
       # FIXME: We build runtimes for both i686 and i386, as "clang -m32" may
       # target different variant than "$CMAKE_C_COMPILER -m32". This part should
       # be gone after we resolve PR14109.
-      test_target_arch(i686 __i686__ "-m32")
+      # Use -march to make sure Clang defines __i686__; see PR24222.
+      test_target_arch(i686 __i686__ "-m32" "-march=i686")
       test_target_arch(i386 __i386__ "-m32")
     else()
       if (CMAKE_SIZEOF_VOID_P EQUAL 4)
         test_target_arch(i386 "" "")
       else()
         test_target_arch(x86_64 "" "")
       endif()
     endif()

compiler-rt/trunk/test/sanitizer_common/TestCases/Linux/open_memstream.cc

-// RUN: %clangxx -m64 -O0 -g -xc++ %s -o %t && %run %t
-// RUN: %clangxx -m64 -O3 -g -xc++ %s -o %t && %run %t
+// RUN: %clangxx -m64 -march=x86-64 -O0 -g -xc++ %s -o %t && %run %t
+// RUN: %clangxx -m64 -march=x86-64 -O3 -g -xc++ %s -o %t && %run %t
 // REQUIRES: x86_64-supported-target
 
 #include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 
 #ifndef __has_feature