[libcxxabi] [cmake] Fix a mismatched variable name

The variable name checked didn't match the one set on the line above.

This error was introduced in b3df14b6c98702ce50401fd039852787373e4676.

GitOrigin-RevId: bf1b81d076f89bd56e86189b013f27dcf4d73ae8
diff --git a/cmake/Modules/HandleLibcxxabiFlags.cmake b/cmake/Modules/HandleLibcxxabiFlags.cmake
index bf5c5fa..98b455c 100644
--- a/cmake/Modules/HandleLibcxxabiFlags.cmake
+++ b/cmake/Modules/HandleLibcxxabiFlags.cmake
@@ -192,7 +192,7 @@
   foreach(flag ${ARGN})
       mangle_name("${flag}" flagname)
       check_c_compiler_flag("${flag}" "C_SUPPORTS_${flagname}_FLAG")
-      add_compile_flags_if(CXX_SUPPORTS_${flagname}_FLAG ${flag})
+      add_compile_flags_if(C_SUPPORTS_${flagname}_FLAG ${flag})
   endforeach()
 endmacro()