contrib/plugins: protect execlog's last_exec expansion

contrib/plugins: protect execlog's last_exec expansion

We originally naively treated expansion as safe because we expected
each new CPU/thread to appear in order. However the -M raspi2 model
triggered a case where a new high cpu_index thread started executing
just before a smaller one.

Clean this up by converting the GArray into the simpler GPtrArray and
then holding a lock for the expansion.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Alexandre Iooss <erdnaxe@crans.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20221027183637.2772968-29-alex.bennee@linaro.org>