Compile `test_num_f128` conditionally on `reliable_f128_math` config
With #132434 merged, our internal SGX CI started failing with:
```
05:27:34 = note: rust-lld: error: undefined symbol: fmodl
05:27:34 >>> referenced by arith.rs:617 (core/src/ops/arith.rs:617)
05:27:34 >>> /home/jenkins/workspace/rust-sgx-ci/rust/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-fortanix-unknown-sgx/release/deps/std-5d5f11eb008c9091.std.d8141acc61ab7ac8-cgu.10.rcgu.o:(std::num::test_num::h7dd9449f6c01fde8)
05:27:34 >>> did you mean: fmodf
05:27:34 >>> defined in: /home/jenkins/workspace/rust-sgx-ci/rust/build/x86_64-unknown-linux-gnu/stage1-std/x86_64-fortanix-unknown-sgx/release/deps/libcompiler_builtins-0376f439a2ebf305.rlib(compiler_builtins-0376f439a2ebf305.compiler_builtins.c22db39d25d6f802-cgu.148.rcgu.o)
```
This originated from the `test_num_f128` test not having the required conditional compilation. This PR fixes that issue.
cc: ````@jethrogb,```` ````@workingjubilee````
c9009ae8c8