libspa's static_dict! only works on x86_64 and x86

On Fedora 35 and Rawhide, on all architectures except x86 and x86_64, I'm getting compilation errors in libspa and pipewire unless I disable tests that make use of static_dict!. This happens on armv7hl, aarch64, ppc64le, and s390x.

error[E0308]: mismatched types
   --> src/dict.rs:412:26
    |
412 |                       key: concat!($k, "\0").as_ptr() as *const i8,
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
...
495 |           let dict = static_dict! {
    |  ____________________-
496 | |             "K0" => "V0",
497 | |             "K1" => "V1"
498 | |         };
    | |_________- in this macro invocation
    |
    = note: expected raw pointer `*const u8`
               found raw pointer `*const i8`
    = note: this error originates in the macro `static_dict` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0308]: mismatched types
   --> src/dict.rs:413:28
    |
413 |                       value: concat!($v, "\0").as_ptr() as *const i8
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
...
495 |           let dict = static_dict! {
    |  ____________________-
496 | |             "K0" => "V0",
497 | |             "K1" => "V1"
498 | |         };
    | |_________- in this macro invocation
    |
    = note: expected raw pointer `*const u8`
               found raw pointer `*const i8`
    = note: this error originates in the macro `static_dict` (in Nightly builds, run with -Z macro-backtrace for more info)