Semihosting: Coalesce message parts into one write syscall

[dnadlinger]

It would be useful to have the option for Semihosting to combine the different message parts into a single temporary buffer before calling WRITE. This would greatly decrease the pain factor due to the long roundtrip latency.

[DoumanAsh]

This should be possible with some static buffer in InterruptFree mode, I'm not sure how safe it would be to have such buffer otherwise

P.s. I'm very busy recently, so do note I'm unlikely to work on it any time soon, but PRs are welcome