powerpc64le truncation failure

[tgross35]

New failure in CI from a recent nightly:

---- float_trunc_f128_ppc::conv stdout ----
thread 'float_trunc_f128_ppc::conv' panicked at testcrate/tests/conv.rs:397:9:
__truncdfhf2(-0.0): apfloat: 0x8000, builtins: 0x0000
stack backtrace:
   0:       0x400009c9dc - <std::sys::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hc338212662bdda50
   1:       0x40000cd188 - core::fmt::write::h01f1102db8e5a9dd
   2:       0x4000099208 - std::io::Write::write_fmt::h228b361863cf408a
   3:       0x400009c758 - std::sys::backtrace::print::h2f115a21d38e781e
   4:       0x400009e990 - std::panicking::default_hook::{{closure}}::h95eb9e64c224ff3f
   5:       0x400009e45c - std::panicking::default_hook::h21a7bae03f381369
   6:       0x400006dc24 - core::ops::function::Fn::call::hcc057380093fc517
   7:       0x40000606ec - test::test_main::{{closure}}::hbe0b5c62ebdcbc3b
   8:       0x400009f474 - std::panicking::rust_panic_with_hook::hd009bce1c85e9d52
   9:       0x400009ef5c - std::panicking::begin_panic_handler::{{closure}}::h4d657c92cff659f4
  10:       0x400009d070 - std::sys::backtrace::__rust_end_short_backtrace::h53f627a96bf5ae39
  11:       0x400009ea34 - rust_begin_unwind
  12:       0x4000014594 - core::panicking::panic_fmt::h924d6e9372f01d25
  13:       0x400001bff0 - testcrate::fuzz_float::h674a5e5f9620c8af
  14:       0x4000017c8c - core::ops::function::FnOnce::call_once::hd83712f1cba592cf
  15:       0x40000655a4 - test::__rust_begin_short_backtrace::ha52be0b4f1ef34c7
  16:       0x4000064d14 - test::run_test::{{closure}}::h3493a58cdca5b3a1
  17:       0x400002493c - std::sys::backtrace::__rust_begin_short_backtrace::h08915bcee01b6ab1
  18:       0x4000029234 - core::ops::function::FnOnce::call_once{{vtable.shim}}::haa7628ec8dd1796b
  19:       0x40000a4208 - std::sys::pal::unix::thread::Thread::new::thread_start::h543c029955df6f48
  20:       0x40011b48b8 - start_thread

Possibly related to #617

[tgross35]

Verified good at rustc 1.80.0-nightly (debd22da6 2024-05-29), verified bad at rustc 1.81.0-nightly (c1b336cb6 2024-06-21)