Allow benchmark allocator features together

[xudong963]

Which issue does this PR close?

Split from #21637.

Rationale for this change

The benchmark binaries currently reject enabling both snmalloc and mimalloc. However, workspace-wide checks such as cargo clippy --all-targets --all-features enable both feature flags, which makes these binaries fail before clippy can run.

What changes are included in this PR?

This PR removes the explicit compile error from the benchmark binaries and makes allocator selection deterministic: snmalloc is used when enabled, otherwise mimalloc is used when enabled.

Are these changes tested?

Are there any user-facing changes?

No. This only affects benchmark binary feature combinations used by development checks.

[kosiew]

@xudong963
Thanks for working on this.

I think you need to amend
benchmarks/benches/sql.rs
too.

cargo check -p datafusion-benchmarks --all-targets --all-features

fails with

error: feature "snmalloc" and feature "mimalloc" cannot be enabled at the same time
  --> benchmarks/benches/sql.rs:47:1
   |
47 | / compile_error!(
48 | |     "feature \"snmalloc\" and feature \"mimalloc\" cannot be enabled at the same time"
49 | | );
   | |_^

error[E0428]: the name `ALLOC` is defined multiple times
  --> benchmarks/benches/sql.rs:57:1
   |
53 | static ALLOC: snmalloc_rs::SnMalloc = snmalloc_rs::SnMalloc;
   | ------------------------------------------------------------ previous definition of the value `ALLOC` here
...
57 | static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;

[xudong963]

@kosiew nice finding, fixed all

[kosiew]

looks 👍 to me