[Discussion] Integrating port specific syscall proxies

With the `risc-v/litex` port, we're able to access the TIME and TIMEH CSRs in usermode. I would like take advantage of this feature to replace the proxies for syscalls, such as `timer_gettime`, with an equivalent implementation that avoids a context switch, where appropriate.

I understand that the `syscall.csv` database is used to generate the syscall proxies and stubs, but there doesn't seem to be any facility to provide a different implementation. Potentially, I could remove the desired syscalls from `syscall.csv` and provide my own proxy which is linked into `libproxies`, but it would be nice if this was more generally supported, rather than patching the kernel locally.

I'd be interested to know if anyone has had a similar idea, or potentially there is a method to achieve this in NuttX which I have overlooked.