lttng-tools: Disable on qemuriscv32

lttng-tools: Disable on qemuriscv32

lttng-tools requires SYS_ppoll and SYS_pselect6 which are not supported on
riscv32. This has been confirmed by lttng-tools upstream.
https://github.com/lttng/lttng-tools/pull/162

It's also turned off for riscv32 in meta-riscv.
https://github.com/riscv/meta-riscv/blob/master/conf/layer.conf

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>