feat(api-caller): Hard-disable all IRQs during API calls

Disable all maskable interrupts on core 1 during API calls.  This brings
two main advantages:

1. It means API calls are now always ISR-safe and can be used everywhere
   in core 1 code.  This is mostly interesting to l0dables as Pycardium
   should not need to do this.

2. It allows Epicardium to halt the clock for core 1 without fear as we
   have observed problems with doing this when core 1 is currently
   executing instructions that touch memory.  Now a synchronous call
   from core 1 will guarantee that it is currently waiting in a WFE and
   no other ISRs could be potentially running.

epicardium/api/caller.c

@@ -5,8 +5,17 @@
 #define MXC_ASSERT_ENABLE
 #include "mxc_assert.h"
 
+static uint32_t irq_save = 0;
+
 void *_api_call_start(api_id_t id, uintptr_t size)
 {
+	/*
+	 * Disable all maskable interrupts here, to be turned on again at the
+	 * end of _api_call_transact().
+	 */
+	irq_save = __get_PRIMASK();
+	__set_PRIMASK(1);
+
 	while (SEMA_GetSema(_API_SEMAPHORE) == E_BUSY) {
 	}
 
@@ -51,6 +60,12 @@ void *_api_call_transact(void *buffer)
 	API_CALL_MEM->call_flag = _API_FLAG_IDLE;
 	SEMA_FreeSema(_API_SEMAPHORE);
 
+	/*
+	 * Re-enable interrupts (if previously enabled) after completing the API
+	 * call.
+	 */
+	__set_PRIMASK(irq_save);
+
 	return API_CALL_MEM->buffer;
 }