re-enable -Wunreachable-code under clang

whitelist the valid cases showing up

If an earlier version of clang is showing more cases (particularly
if they don't make sense), the re-enabling of the warning in
sys/unix/hints/include/compiler.2020 can be made clang-version
specific instead. I had no way to test earlier versions.

util/panic.c

@@ -23,6 +23,7 @@ boolean panicking;
 void panic(const char *, ...);
 
 DISABLE_WARNING_FORMAT_NONLITERAL
+DISABLE_WARNING_UNREACHABLE_CODE
 
 void panic
 VA_DECL(const char *, str)
@@ -45,9 +46,11 @@ VA_DECL(const char *, str)
         abort(); /* generate core dump */
 #endif
     VA_END();
+    /* UNREACHABLE_CODE */
     exit(EXIT_FAILURE); /* redundant */
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
 RESTORE_WARNING_FORMAT_NONLITERAL
 
 #ifdef ALLOCA_HACK