`extern "C" fn` on mips64 targets does not respect repr(transparent)

[RalfJung]

This is the MIPS version of #115336: on the target mips64-unknown-linux-gnuabi64, all arrays and some tuple types like (i32, f32, i64, f64) have a different ABI when being wrapped in repr(transparent) wrappers, and hence this fails the test I added in #115372.

For instance for [u8; 2], the PassMode becomes

                   mode: Indirect {
                       attrs: ArgAttributes {
                           regular: NoAlias | NoCapture | NonNull | NoUndef,
                           arg_ext: None,
                           pointee_size: Size(2 bytes),
                           pointee_align: Some(
                               Align(1 bytes),
                           ),
                       },
                       extra_attrs: None,
                       on_stack: false,
                   },

But for Wrapper<[u8; 2]> it becomes

                   mode: Cast(
                       CastTarget {
                           prefix: [
                               None,
                               None,
                               None,
                               None,
                               None,
                               None,
                               None,
                               None,
                           ],
                           rest: Uniform {
                               unit: Reg {
                                   kind: Integer,
                                   size: Size(8 bytes),
                               },
                               total: Size(2 bytes),
                           },
                           attrs: ArgAttributes {
                               regular: (empty),
                               arg_ext: None,
                               pointee_size: Size(0 bytes),
                               pointee_align: None,
                           },
                       },
                       false,
                   ),

This also affects mips64el-unknown-linux-gnuabi64, mips64-openwrt-linux-musl, mips64-unknown-linux-muslabi64, mips64el-unknown-linux-muslabi64
Cc @Itus-Shield (listed as maintainer for the openwrt target)

[RalfJung]

As per #115238, all affected targets are tier 3 targets.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-low