gc.c: rename mrb_alloca() to mrb_temp_alloc() and fix memory leaks

rename mrb_alloca() to mrb_temp_alloc() for clearer naming - the new name
better describes its purpose as GC-managed temporary allocation. keep
mrb_alloca() as a macro alias for backward compatibility.

apply mrb_temp_alloc() to fix potential memory leaks in:
- mruby-strftime: if mrb_str_cat() raises, allocated buffers now cleaned by GC
- mruby-io File.readlink: if mrb_str_new() raises, buffer now cleaned by GC

Co-authored-by: Claude <noreply@anthropic.com>

src/gc.c

@@ -266,7 +266,7 @@ mrb_free(mrb_state *mrb, void *p)
 }
 
 MRB_API void*
-mrb_alloca(mrb_state *mrb, size_t size)
+mrb_temp_alloc(mrb_state *mrb, size_t size)
 {
   struct RString *s;
   s = MRB_OBJ_ALLOC(mrb, MRB_TT_STRING, NULL);