buildman: README: add links for toolchains not available on kernel.org

Add links for toolchains not available on kernel.org.

The sh4 toolchains from kernel.org dose not work for some boards,
so use the sh from Sourcery.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>