config.mk: Added variants of `valopt`/`opt` that do not automatically `putvar`.

Used aforementioned variants to extract options that have explicit
`putvar` calls associated with them in the subsequent code.  When the
explicit `putvar` call was conditional on some potentially complex
condition, moved the `putvar` call out to the main control flow of the
script so that it always runs if necessary.

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As a driveby fix, captured the error exit when doing the test run of
`rustc --version` from `CFG_LOCAL_RUST_ROOT`, and signal explicit
configure failure when it did not run successfully.  (If we cannot run
`rustc`, we really shouldn't try to keep going.)

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Finally, in response to review feedback, went through and identified
cases where we had been calling `putvar` manually (and thus my naive
translation used `opt_nosave`/`valopt_nosave`), and then verified
whether a manual `putvar` was necessary (i.e., was each variable in
question manually computed somewhere in the `configure` script).
In cases that did not meet this criteria, I revised the code to use
the `opt`/`valopt` directly and removed the corresponding `putvar`,
cleaning things up a teeny bit.

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Fix #17887.