Fix early timer interrupt causing exception before first task launch

In preemptive scheduling mode, enabling the machine timer interrupt too
early can trigger an interrupt before the first task is launched,
resulting in an instruction access fault. This happens because
hal_timer_enable() enables MTIE before a valid mtimecmp value is set.

Set mtimecmp before enabling the timer interrupt to ensure the first
interrupt is scheduled in the future, avoiding premature preemption and
exceptions in user applications like cpubench.

Author: visitorckw

arch/riscv/hal.c

@@ -308,6 +308,7 @@ void do_trap(uint32_t cause, uint32_t epc)
 /* Enables the machine-level timer interrupt source. */
 void hal_timer_enable(void)
 {
+    mtimecmp_w(mtime_r() + (F_CPU / F_TIMER));
     write_csr(mie, read_csr(mie) | MIE_MTIE);
 }