19157: sys/shell_lock: do not call strlen, less jumpy r=benpicco a=kfessel
### Contribution description
the current `_safe_strcmp` depends on not being optimized and not being inlined (implicitly given by the -O0), this new one does less (would say not since even O3 had compile results that should not return early or show different runtimes for different secrets).
the runtime of strlen depend on the length of the string (password) therefor it is removed. `ifs` are very jumpy and depend on the contend of password, this avoids `ifs` sadly clang still compiles some boolean statements to if.
with this PR the `__attribute__((optimize("O0")))` should be removable.
### Testing procedure
see https://godbolt.org/z/x35b85cEx
and run the `<RIOT>/tests/shell_lock`
### Issues/PRs references
#19155 made me aware of this function
Co-authored-by: Karl Fessel <karl.fessel@ovgu.de>
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