RISC-V: disallow gcc + rust builds

During the discussion before supporting rust on riscv, it was decided
not to support gcc yet, due to differences in extension handling
compared to llvm (only the version of libclang matching the c compiler
is supported). Recently Jason Montleon reported [1] that building with
gcc caused build issues, due to unsupported arguments being passed to
libclang. After some discussion between myself and Miguel, it is better
to disable gcc + rust builds to match the original intent, and
subsequently support it when an appropriate set of extensions can be
deduced from the version of libclang.

Closes: https://lore.kernel.org/all/20240917000848.720765-2-jmontleo@redhat.com/ [1]
Link: https://lore.kernel.org/all/20240926-battering-revolt-6c6a7827413e@spud/

 [2]
Fixes: 70a57b24 ("RISC-V: enable building 64-bit kernels with rust support")
Cc: stable@vger.kernel.org
Reported-by: Jason Montleon <jmontleo@redhat.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20241001-playlist-deceiving-16ece2f440f5@spud


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>