toolchain.mk: Update legacy toolchain download url

Fixes https://github.com/OP-TEE/optee_os/issues/1195

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Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org>