ignore x86-stdcall on mingw

Author: pietroalbini

tests/ui/symbol-names/x86-stdcall.rs

@@ -1,6 +1,7 @@
 // build-pass
 // only-x86
 // only-windows
+// ignore-gnu - vectorcall is not supported by GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89485
 #![crate_type = "cdylib"]
 #![feature(abi_vectorcall)]