scripts: prevent GCC optimising test symbols in test-symbol-check #22645

I noticed in #22381 that when the test-symbol-check target was being built with Clang and run in the CI it would fail due to using a too-new version of pow (used here). Our CIs use Focal (glibc 2.31) and the version of pow was the optimized version introduced in glibc 2.29:

* Optimized generic exp, exp2, log, log2, pow, sinf, cosf, sincosf and tanf.

This made sense, except for that if it was failing when built using Clang, why hadn't it also been failing when being built with GCC?

Turns out GCC is optimizing away that call to pow at all optimization levels, including -O0, see: https://godbolt.org/z/53MhzMxT7, and this has been the case forever, or at least since GCC 5.x. Clang on the other hand, will only optimize away the pow call at -O1 and -O2, not -O0: https://godbolt.org/z/Wbnqj3q6c. Thus when this test was built with Clang (we don't pass -O so we default to -O0) it was failing in the CI environment, because it would actually have a call to the "new" pow.

Avoid this issue by using a symbol that won't be optimized away, or that we are unlikely to ever have versioning issues with.

Looks like the following comment should be updated as well https://github.com/bitcoin/bitcoin/blob/b6f32d03b0ad20f46af915e892a31e42187893c4/contrib/devtools/test-symbol-check.py#L76

In https://github.com/bitcoin/bitcoin/blob/b6f32d03b0ad20f46af915e892a31e42187893c4/contrib/devtools/test-symbol-check.py#L90 why -lm, if pow has been dropped?

why -lm, if pow has been dropped? Looks like the following commit should be updated as well

Addressed.

Guix builds:

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ACK 5449d44e37982fcd5251fd47873c5f7d34c39fc9