fix: make vPortYield weak in ARM_CRx_No_GIC port (#1414)

Mark vPortYield as a weak symbol so chips with a dedicated software
interrupt register can substitute their own yield trigger.

Default behaviour is unchanged when no strong override is linked.

Signed-off-by: Maxim De Clercq <maximdeclercq00@gmail.com>

Author: maximdeclercq

portable/GCC/ARM_CRx_No_GIC/portASM.S

@@ -50,7 +50,7 @@
     .global vPortRestoreTaskContext
     .global vPortInitialiseFPSCR
     .global ulReadAPSR
-    .global vPortYield
+    .weak   vPortYield
     .global vPortEnableInterrupts
     .global vPortDisableInterrupts
     .global ulPortSetInterruptMaskFromISR