bootstrap: untangle static-libstdcpp & llvm-tools

Previously, the static-libstdcpp setting was tied to llvm-tools such
that enabling the latter always enabled the latter. This seems
unfortunate, since it is entirely reasonable for someone to want to
_not_ statically link stdc++, but _also_ want to build the llvm-tools.
This patch therefore separates the two settings such that neither
implies the other.

On its own, that would change the default behavior in a way that's
likely to surprise users. Specifically, users who build llvm-tools
_likely_ want those tools to be statically compiled against libstdc++,
since otherwise users with older GLIBCXX will be unable to run the
vended tools. So, we also flip the default for the `static-libstdcpp`
setting such that builds always link statically against libstdc++ by
default, but it's _possible_ to opt out.

See also #94719.

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