Use the correct char type on all platforms

tests/ui/const-generics/issues/issue-72352.full.stderr

@@ -1,7 +1,7 @@
 error[E0741]: using function pointers as const generic parameters is forbidden
-  --> $DIR/issue-72352.rs:7:42
+  --> $DIR/issue-72352.rs:8:42
    |
-LL | unsafe fn unsafely_do_the_thing<const F: fn(&CStr) -> usize>(ptr: *const i8) -> usize {
+LL | unsafe fn unsafely_do_the_thing<const F: fn(&CStr) -> usize>(ptr: *const c_char) -> usize {
    |                                          ^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error

tests/ui/const-generics/issues/issue-72352.min.stderr

@@ -1,7 +1,7 @@
 error: using function pointers as const generic parameters is forbidden
-  --> $DIR/issue-72352.rs:7:42
+  --> $DIR/issue-72352.rs:8:42
    |
-LL | unsafe fn unsafely_do_the_thing<const F: fn(&CStr) -> usize>(ptr: *const i8) -> usize {
+LL | unsafe fn unsafely_do_the_thing<const F: fn(&CStr) -> usize>(ptr: *const c_char) -> usize {
    |                                          ^^^^^^^^^^^^^^^^^^
    |
    = note: the only supported types are integers, `bool` and `char`

tests/ui/const-generics/issues/issue-72352.rs

@@ -1,10 +1,11 @@
 // revisions: full min
+
 #![cfg_attr(full, feature(adt_const_params))]
 #![cfg_attr(full, allow(incomplete_features))]
 
-use std::ffi::{CStr, CString};
+use std::ffi::{c_char, CStr, CString};
 
-unsafe fn unsafely_do_the_thing<const F: fn(&CStr) -> usize>(ptr: *const i8) -> usize {
+unsafe fn unsafely_do_the_thing<const F: fn(&CStr) -> usize>(ptr: *const c_char) -> usize {
     //~^ ERROR: using function pointers as const generic parameters is forbidden
     F(CStr::from_ptr(ptr))
 }
@@ -16,7 +17,5 @@ fn safely_do_the_thing(s: &CStr) -> usize {
 fn main() {
     let baguette = CString::new("baguette").unwrap();
     let ptr = baguette.as_ptr();
-    println!("{}", unsafe {
-        unsafely_do_the_thing::<safely_do_the_thing>(ptr)
-    });
+    println!("{}", unsafe { unsafely_do_the_thing::<safely_do_the_thing>(ptr) });
 }