[compiler-rt] Fix building on OpenBSD/amd64

OpenBSD/amd64 does not use multi-lib. Enabling the CET support on amd64
exposed that CMake was using a multi-lib build on amd64.

Author: brad0

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

@@ -213,7 +213,7 @@ macro(test_targets)
     if(COMPILER_RT_DEFAULT_TARGET_ONLY)
       add_default_target_arch(${COMPILER_RT_DEFAULT_TARGET_ARCH})
     elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "i[2-6]86|x86|amd64")
-      if(NOT MSVC)
+      if(NOT MSVC AND NOT "${CMAKE_SYSTEM_NAME}" STREQUAL "OpenBSD")
         test_target_arch(x86_64 "" "-m64")
         test_target_arch(i386 __i386__ "-m32")
       else()