project: Fix support for explicit RC language

The check added in commit v3.6.0-rc1~293^2 (Diagnose recursive
project/enable_language without crashing, 2016-03-07) broke support for
enabling `RC` explicitly along with other languages like `C`.  The
reason is that we enable all listed languages at once so the internal
`enable_language(RC)` that we do while enabling `C` or `CXX` on some
platforms triggers the recursion check if `RC` is explicitly listed.

Ideally we should refactor things to only enable one language at a time,
but for now it is simplest to just exclude `RC` from the explicit list
until other languages are enabled, and then enable it.

Closes: #16330
1 job for fix-explicit-RC
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