libroot/riscv64: define syscall function size, implement time

Change-Id: Ib08b0b700506e3e1eb41d1b30b025bdf688342d9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4071
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

libroot/os: implement arch-specific parts for riscv64

Change-Id: Idb6920b4c98a579a184e6a7e1976f2d8c7618dba
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3983
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>