Change variadic-ffi-2 to use a platform independant ABI

Otherwise this test will include a future incompatibility warning
on some targets but not others.

Author: tdittr

tests/ui/c-variadic/variadic-ffi-2.rs

@@ -1,9 +1,6 @@
-//@ ignore-arm stdcall isn't supported
-//@ ignore-x86 stdcall isn't supported
 #![feature(extended_varargs_abi_support)]
 
-#[allow(unsupported_fn_ptr_calling_conventions)]
-fn baz(f: extern "stdcall" fn(usize, ...)) {
+fn baz(f: extern "Rust" fn(usize, ...)) {
     //~^ ERROR: C-variadic function must have a compatible calling convention,
     // like C, cdecl, system, aapcs, win64, sysv64 or efiapi
     f(22, 44);

tests/ui/c-variadic/variadic-ffi-2.stderr

@@ -1,19 +1,9 @@
 error[E0045]: C-variadic function must have a compatible calling convention, like `C`, `cdecl`, `system`, `aapcs`, `win64`, `sysv64` or `efiapi`
-  --> $DIR/variadic-ffi-2.rs:6:11
+  --> $DIR/variadic-ffi-2.rs:3:11
    |
-LL | fn baz(f: extern "stdcall" fn(usize, ...)) {
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
+LL | fn baz(f: extern "Rust" fn(usize, ...)) {
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention
 
 error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0045`.
-Future incompatibility report: Future breakage diagnostic:
-warning: the calling convention "stdcall" is not supported on this target
-  --> $DIR/variadic-ffi-2.rs:6:11
-   |
-LL | fn baz(f: extern "stdcall" fn(usize, ...)) {
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #130260 <https://github.com/rust-lang/rust/issues/130260>
-