[SystemZ][z/OS] Account for renamed parameter name (libc++)

The following patch (https://reviews.llvm.org/D129051) broke z/OS builds by renaming the parameter name. This patch accounts for that change.

Differential Revision: https://reviews.llvm.org/D132946

NOKEYCHECK=True
GitOrigin-RevId: e1e9961f7692cfb84c4b3bb213d515d4c80332a4
diff --git a/include/regex b/include/regex
index a3bbc3c..8361b41 100644
--- a/include/regex
+++ b/include/regex
@@ -1355,7 +1355,7 @@
 unsigned char __to_lower(unsigned char __c)
 {
 #if defined(__MVS__) && !defined(__NATIVE_ASCII_F)
-    return c & 0xBF;
+    return __c & 0xBF;
 #else
     return __c | 0x20;
 #endif