Add gounthar to riscv platform team

[gounthar]

@sxa suggested in nodejs/build#4099 (comment) that I open an issue here, similar to #1042, to be added to the platform-riscv64 team so I get notified along with the other RISC-V people.

For context: I run a couple of Banana Pi F3 boards (SpacemiT K1) as self-hosted runners, and I have been following the RISC-V threads in nodejs/build#4099 and nodejs/node#64538. Most of what I can usefully offer is testing on real K1 hardware when that helps, plus whatever digging I can do on the V8 side.

I have only been active in these threads for a few days, so no hard feelings if this is premature.

[sxa]

@nodejs/tsc Not sure if you need tsc-agenda on this but Bruno is someone I've worked with on things in the past and would be a great addition to the RISC-V team so I'd like to see him added to the group of people who can be pinged when issues show up on this platform.