kbuild: rust-analyzer: better error handling

1) Provide a better error message for the "Rust not available" case.
Without this patch, one gets various misleading messages, such as:

    "No rule to make target 'rust-analyzer'"

Instead, run scripts/rust_is_available.sh directly, as a prerequisite,
and let that script report the cause of any problems, as well as
providing a link to the documentation. Thanks to Miguel Ojeda for the
idea of just letting rust_is_available.sh report its results directly.

The new output in the failure case looks like this:

$ make rust-analyzer
***
*** Rust compiler 'rustc' could not be found.
***
***
*** Please see Documentation/rust/quick-start.rst for details
*** on how to set up the Rust support.
***
make[1]: *** [/kernel_work/linux-github/Makefile:1975: rust-analyzer] Error 1
make: *** [Makefile:240: __sub-make] Error 2

Reviewed-by: Finn Behrens <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Tested-by: Alice Ryhl <[email protected]>
Signed-off-by: John Hubbard <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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