kconfig: llvm: riscv: Advertise TLS support when using llvm for RISC-V

clang and lld support TLS for RISC-V, so advertise this support in
Kconfig. I believe other non-RISC-V targets also have TLS support in LLVM,
but I'm not sure on the exact subset. As TLS support in LLVM wasn't
automatically advertised previously, gate this on RISC-V for now as it
a) shouldn't break other targets and b) prevents us from improperly
claiming support for unsupported targets.

Signed-off-by: Jonathon Penix <jpenix@quicinc.com>