Fix SSTC extension support

[br]

Summary

Based on SSTC spec (1):

"If the stimecmp (supervisor-mode timer compare) register is implemented, STIP is read-only in mip and reflects the supervisor-level timer interrupt signal resulting from stimecmp. This timer interrupt signal is cleared by writing stimecmp with a value greater than the current time value."

This fixes operation in Spike with SSTC extension enabled, e.g.

$ spike --isa RV64IMAFDCH_zicntr_zihpm_sstc

[1]. https://tools.cloudbear.ru/docs/riscv-sstc-0.5.4-20211013.pdf

Test Plan

Tested in Spike only

[mhorne]

sys/riscv/riscv/timer.c

147

We can rely on sign-extension or should it be -1UL?

[br]

use UL suffix to not rely on sign-extension