rust: kconfig: Don't require RUST_IS_AVAILABLE for rustc-option

rust: kconfig: Don't require RUST_IS_AVAILABLE for rustc-option

The final version of this macro does not fail in the absence of an
invokable `$(RUSTC)`, so we don't need to be careful not to invoke it.

Link: https://lore.kernel.org/all/CAGSQo01mQfcU1EiW53be1hcts0c1p-HQAab_HBk6VcVmhq3n2Q@mail.gmail.com/
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Link: https://patch.msgid.link/20250909-docrem-v1-1-dcc69059a5cb@google.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>