Use RISC-V Sstc extension

[sbutz]

Enables RustyHermit to manage timers without trapping to m-mode or to the hypervisor.

https://docs.riscv.org/reference/isa/v20260120/priv/sstc.html

This extension serves to provide supervisor mode with its own CSR-based timer interrupt facility that it can directly manage to provide its own timer service (in the form of having its own stimecmp register) - thus eliminating the large overheads for emulating S/HS-mode timers and timer interrupt generation up in M-mode. Further, this extension adds a similar facility to the Hypervisor extension for VS-mode.

As long as RustyHermit supports hardware not compliant with RVA23 profile, we need to support both mechanisms.

Options:

1. Detect availability of Ssct extension at runtime by parsing the fdt.
2. Add feature flag.