[C++][Compute] Make a subset of compute:: available even if ARROW_COMPUTE=OFF

[asfimport]

functions like Cast and Take are internally useful enough to be included in libarrow.so even if ARROW_COMPUTE=OFF

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-13563. Please see the migration documentation for further details.}

[asfimport]

Ben Kietzman / @bkietz:
@pitrou

[github-actions]

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