freebsd: make check bench_bitcoin illegal instruction

Reproduce the failure by running ./src/bench/bench_bitcoin on FreeBSD 14.2 x86_64 and identify which benchmark triggers the illegal instruction after BnBExhaustion. Trace that benchmark into the relevant source and determine the platform or instruction-set cause. Done means the benchmark completes without an illegal-instruction crash and the behavior is covered by an appropriate test or documented reproduction.

OS: FreeBSD 14.2
Tag: Elements-23.2.4
Arch: x86_64

The bitcoind v23.2 bench_bitcoin runs fine.

% ./src/bench/bench_bitcoin 

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|       33,918,912.00 |               29.48 |    0.4% |      0.32 | `AddrManAdd`
|       41,379,012.00 |               24.17 |    0.2% |      0.46 | `AddrManAddThenGood`
|          143,318.71 |            6,977.46 |    0.2% |      0.01 | `AddrManGetAddr`
|              148.36 |        6,740,300.72 |    0.5% |      0.01 | `AddrManSelect`
|          349,355.33 |            2,862.42 |    0.2% |      0.01 | `AssembleBlock`

|             ns/byte |              byte/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|               68.69 |       14,557,263.45 |    0.7% |      0.01 | `Base58CheckEncode`
|               15.00 |       66,656,073.03 |    0.3% |      0.01 | `Base58Decode`
|               50.84 |       19,670,034.41 |    0.3% |      0.01 | `Base58Encode`
|                7.09 |      141,086,434.00 |    0.1% |      0.01 | `Bech32Decode`
|               15.34 |       65,201,841.52 |    0.2% |      0.01 | `Bech32Encode`

|               ns/op |                op/s |    err% |     total | benchmark
|--------------------:|--------------------:|--------:|----------:|:----------
|              134.03 |        7,461,269.48 |    0.8% |      0.01 | `BenchLockedPool`
|               28.87 |       34,637,621.14 |    0.0% |      0.01 | `BenchTimeDeprecated`
|               32.33 |       30,929,578.06 |    0.1% |      0.01 | `BenchTimeMillis`
|               32.31 |       30,953,487.11 |    0.2% |      0.01 | `BenchTimeMillisSys`
|                2.30 |      433,852,393.89 |    0.0% |      0.01 | `BenchTimeMock`
|       53,562,096.00 |               18.67 |    0.2% |      0.59 | `BlockToJsonVerbose`
|       33,862,387.00 |               29.53 |    0.3% |      0.38 | `BlockToJsonVerboseWrite`
|          677,365.50 |            1,476.31 |    0.1% |      0.01 | `BnBExhaustion`
Illegal instruction (core dumped)