better awareness of unwind tables

* stage1 backend allows configuring the uwtables function attr
   via a flag rather than its own logic.
 * stage2 defaults to enabling uwtable attr when
   linking libunwind, or always on windows
 * stage2 makes link_eh_frame_hdr true automatically if uwtable
   attr is set to be on for zig functions
 * CLI: add -funwind-tables and -fno-unwind-tables to allow the user to
   override the defaults.
 * hook it up to `zig cc`

closes #9046

src/stage1/codegen.cpp

@@ -211,7 +211,7 @@ static ZigLLVM_CallingConv get_llvm_cc(CodeGen *g, CallingConvention cc) {
 }
 
 static void add_uwtable_attr(CodeGen *g, LLVMValueRef fn_val) {
-    if (g->zig_target->os == OsWindows) {
+    if (g->unwind_tables) {
         addLLVMFnAttr(fn_val, "uwtable");
     }
 }