fix: use luatexbase to register pgfsys@strcmp

[hmenke]

Motivation for this change

Workaround for latex3/latex2e#1100

Checklist

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• Code changes are licensed under GPLv2 + LPPLv1.3c
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[muzimuzhi]

The logic looks OK to me, but overall my LuaTeX knowledge is not enough to review this PR.

As for l3kernel, it defines \tex_strcmp:D for LuaTeX using its own wrapper luacmd(name, func, ...), see here. Checking how luacmd() is defined, it loads ltluatex module if none of context and luatexbase are loaded and has a special branch if context module is loaded.

Linking to #1362 and don't forget to add a Changelog entry.