os.execShellCmd: use WEXITSTATUS to retrieve exit code (#10222)

According to POSIX, system() shall returns the termination status in the
format specified by waitpid(), which means WEXITSTATUS should be used to
retrieve the exit code portably.

This fixes execShellCmd on Haiku.

lib/pure/os.nim

@@ -1298,7 +1298,7 @@ proc execShellCmd*(command: string): int {.rtl, extern: "nos$1",
   ## shell involved, use the `execProcess` proc of the `osproc`
   ## module.
   when defined(posix):
-    result = c_system(command) shr 8
+    result = WEXITSTATUS(c_system(command))
   else:
     result = c_system(command)