YJIT: Protect strings from GC on String#<< (#7466)

Fix https://github.com/Shopify/yjit/issues/310

[Bug #19483]

Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Co-authored-by: Jimmy Miller <jimmy.miller@shopify.com>

yjit/src/codegen.rs

@@ -4288,6 +4288,9 @@ fn jit_rb_str_concat(
     // Guard that the argument is of class String at runtime.
     let arg_type = ctx.get_opnd_type(StackOpnd(0));
 
+    // Guard buffers from GC since rb_str_buf_append may allocate.
+    gen_save_sp(asm, ctx);
+
     let concat_arg = ctx.stack_pop(1);
     let recv = ctx.stack_pop(1);