freertos: ensure interrupts are disabled before enabling tick timer

xPortStartScheduler calls vPortSetupTimer -> _frxt_tick_timer_init,
which enables tick timer interrupt and sets up the first timeout.
From that point on, the interrupt can fire. If the interrupt happens
while _frxt_dispatch is running, the scheduler will enter an infinite
loop. This is because _frxt_dispatch isn't supposed to be preemptable,
and the tick interrupt will overwrite some of the registers used by
_frxt_dispatch.
Note that this situation doesn't practically occur on the real
hardware, where the execution of vPortSetupTimer and _frxt_dispatch
happens quickly enough. However it can be reproduced on an emulator
if the tick period is set to 1ms.

Add an explicit call to portDISABLE_INTERRUPTS in xPortStartScheduler
to guarantee that _frxt_dispatch doesn't run with interrupts enabled.
This is similar to the esprv_intc_int_set_threshold(1); call in
RISC-V version of port.c.

Author: igrr

components/freertos/FreeRTOS-Kernel/portable/xtensa/port.c

@@ -94,6 +94,7 @@ extern void _xt_coproc_init(void);
 
 BaseType_t xPortStartScheduler( void )
 {
+    portDISABLE_INTERRUPTS();
     // Interrupts are disabled at this point and stack contains PS with enabled interrupts when task context is restored
 
 #if XCHAL_CP_NUM > 0