Non-conforming `printf` prototypes in the GCC/Rust testsuite

Just for fun, I tried enabling GCC/Rust in my GCC/nvptx target testing (see https://gcc.gnu.org/wiki/nvptx).

As you'd hope, it generally works.

One issue, however, is that in our hacked-up printf test cases, we use:

extern "C" {
    fn printf(s: *const i8, ...);
}

..., which GCC/nvptx turns into:

.extern .func printf (.param.u64 %in_ar0, .param.u64 %in_ar1);

In contrast, the libc one is declared/defined as:

.visible .func (.param .u32 %value_out) printf (.param .u64 %in_ar0, .param .u64 %in_ar1);

(That is, with the expected int return type; don't ask me why unsigned .u32 instead of signed .s32, but apparently that's OK.)

Therefore, all those execution tests FAIL:

error   : Prototype doesn't match for 'printf' [...]
nvptx-run: cuLinkAddData failed: unknown error (CUDA_ERROR_UNKNOWN, 999)

Should I fix all those up like (a):

 extern "C" {
-   fn printf(s: *const i8, ...);
+   fn printf(s: *const i8, ...) -> i32;
 }

..., which should be correst most of all times, or (b):

+pub type c_int = i32;
 extern "C" {
-   fn printf(s: *const i8, ...);
+   fn printf(s: *const i8, ...) -> c_int;
 }

..., to make that more obvious, or (c) something else?

(See https://doc.rust-lang.org/stable/std/ffi/type.c_int.html.)

Start by locating the GCC/Rust testsuite execution cases that declare the variadic printf function and compare them with the libc declaration used by GCC/nvptx. Confirm the appropriate return type and update the affected declarations so the nvptx prototypes match, then rerun the execution tests and verify that the printf prototype errors are gone.