YJIT: Use #[cfg] instead of if cfg!

[k0kubun]

Rust 1.70.0 came out and it seems to have broken our build. This is the same patch as #7893 for ruby_3_2 branch.

  = note: /usr/bin/ld: /tmp/cirrus-ci-build/yjit/target/debug/deps/yjit-0044bace7c3a8bfe.5b58a5nco7jhlap8.rcgu.o: in function `rb_yjit_record_exit_stack':
          /tmp/cirrus-ci-build/yjit/src/stats.rs:646: undefined reference to `rb_vm_insn_addr2opcode'
          /usr/bin/ld: /tmp/cirrus-ci-build/yjit/src/stats.rs:662: undefined reference to `rb_profile_frames'
          collect2: error: ld returned 1 exit status

if cfg! seems not reliable to avoid linking functions in the block, and apparently it stopped working in Rust 1.70.0. #[cfg] seems like the proper way to do it, so this PR uses it instead.