Increase the benchmark rustc version to 2025-05-28

[tgross35]

We may soon want to use some new nightly features in compiler-builtins
and libm, specifically cfg_target_has_reliable_f16_f128 which was
added in the past few weeks. This will mean we need a newer toolchain
for benchmarks to continue building.

Bump to the current latest nightly so we are not blocked on this down
the line.

ci: allow-regressions

[tgross35]

Pretty uniform 1-5% improvements across the board since this comes with an LLVM version bump. E.g.

count::icount_bench_copysign_group::icount_bench_copysign logspace:setup_copysign()
  Baselines:                      softfloat|softfloat
  Instructions:                        5492|7607                 (-27.8033%) [-1.38511x]
  L1 Hits:                             8755|10868                (-19.4424%) [-1.24135x]
  L2 Hits:                                2|3                    (-33.3333%) [-1.50000x]
  RAM Hits:                               5|6                    (-16.6667%) [-1.20000x]
  Total read+write:                    8762|10877                (-19.4447%) [-1.24138x]
  Estimated Cycles:                    8940|11093                (-19.4086%) [-1.24083x]
icount::icount_bench_fma_group::icount_bench_fma logspace:setup_fma()
  Baselines:                      softfloat|softfloat
  Instructions:                       51204|53426                (-4.15902%) [-1.04340x]
  L1 Hits:                            57968|60188                (-3.68844%) [-1.03830x]
  L2 Hits:                                4|6                    (-33.3333%) [-1.50000x]
  RAM Hits:                              19|19                   (No change)
  Total read+write:                   57991|60213                (-3.69023%) [-1.03832x]
  Estimated Cycles:                   58653|60883                (-3.66276%) [-1.03802x]

[tgross35]

The copysign difference, for my own curiosity.

.section .text.libm::math::copysign::copysign,"ax",@progbits
        .globl  libm::math::copysign::copysign
        .p2align        4, 0x90
.type   libm::math::copysign::copysign,@function
libm::math::copysign::copysign:
        .cfi_startproc
        andps xmm0, xmmword ptr [rip + .LCPI27_0]
        movsd xmm2, qword ptr [rip + .LCPI27_1]
        andps xmm2, xmm1
        orps xmm0, xmm2
        ret

.section .text.libm::math::copysign::copysign,"ax",@progbits
        .globl  libm::math::copysign::copysign
        .p2align        4
.type   libm::math::copysign::copysign,@function
libm::math::copysign::copysign:
        .cfi_startproc
        andps xmm1, xmmword ptr [rip + .LCPI27_0]
        andps xmm0, xmmword ptr [rip + .LCPI27_1]
        orps xmm0, xmm1
        ret