[libc] Keep headers compatible with -std=c89 mode (llvm#121981)

C89 doesn't have the `restrict` keyword.  When in `-std=c89`
mode, GNU-compatible compilers require the `__restrict` spelling.

libc/include/__llvm-libc-common.h

@@ -50,7 +50,14 @@
 #define __END_C_DECLS
 
 #undef __restrict
-#define __restrict restrict // C99 and above support the restrict keyword.
+#if __STDC_VERSION__ >= 199901L
+// C99 and above support the restrict keyword.
+#define __restrict restrict
+#elif !defined(__GNUC__)
+// GNU-compatible compilers accept the __ spelling in all modes.
+// Otherwise, omit the qualifier for pure C89 compatibility.
+#define __restrict
+#endif
 
 #undef _Noreturn
 #if __STDC_VERSION__ >= 201112L