chore: add additional compiler flags (#17815)

Added:
  - -Wdouble-promotion
  - -Wmissing-noreturn
  - -Wmissing-format-attribute
  - -Wsuggest-attribute={pure,const,malloc,cold}

Resolves: #343

src/nvim/os/signal.c

@@ -152,6 +152,7 @@ static char *signal_name(int signum)
 // NOTE: Avoid unsafe functions, such as allocating memory, they can result in
 // a deadlock.
 static void deadly_signal(int signum)
+  FUNC_ATTR_NORETURN
 {
   // Set the v:dying variable.
   set_vim_var_nr(VV_DYING, 1);