move __utc string object to its own translation unit

having it in __tz.c caused gmtime[_r] and timegm to pull in all of the
time zone code despite having no need for it.

src/time/__tz.c

@@ -24,7 +24,6 @@ weak_alias(__tzname, tzname);
 
 static char std_name[TZNAME_MAX+1];
 static char dst_name[TZNAME_MAX+1];
-const char __utc[] = "UTC";
 
 static int dst_off;
 static int r0[5], r1[5];

src/time/__utc.c

@@ -0,0 +1,3 @@
+#include "time_impl.h"
+
+const char __utc[] = "UTC";