Add support for vApplicationFPUSafeIRQHandler (#1113)

This PR adds support for vApplicationFPUSafeIRQHandler. The application
writer needs to name their IRQ handler as:
1. vApplicationIRQHandler if the IRQ handler does not use FPU registers.
2. vApplicationFPUSafeIRQHandler is the IRQ handler uses FPU registers.

When the application uses vApplicationFPUSafeIRQHandler, a default
implementation of vApplicationIRQHandler is used which stores FPU
registers and then calls vApplicationFPUSafeIRQHandler.

portable/GCC/ARM_CRx_No_GIC/portASM.S

diff --git a/portable/GCC/ARM_CRx_No_GIC/portASM.S b/portable/GCC/ARM_CRx_No_GIC/portASM.S
index 9945fc2..de9845e 100644
--- a/portable/GCC/ARM_CRx_No_GIC/portASM.S
+++ b/portable/GCC/ARM_CRx_No_GIC/portASM.S
@@ -39,6 +39,7 @@
     .extern pxCurrentTCB
     .extern vTaskSwitchContext
     .extern vApplicationIRQHandler
+    .extern vApplicationFPUSafeIRQHandler
     .extern ulPortInterruptNesting
     .extern ulPortTaskHasFPUContext
     .extern ulICCEOIR
@@ -237,6 +238,50 @@
 
 /*-----------------------------------------------------------*/
 
+/* If the application provides an implementation of vApplicationIRQHandler(),
+ * then it will get called directly without saving the FPU registers on
+ * interrupt entry, and this weak implementation of vApplicationIRQHandler()
+ * will not get called.
+ *
+ * If the application provides its own implementation of
+ * vApplicationFPUSafeIRQHandler() then this implementation of
+ * vApplicationIRQHandler() will be called, save the FPU registers, and then
+ * call vApplicationFPUSafeIRQHandler().
+ *
+ * Therefore, if the application writer wants FPU registers to be saved on
+ * interrupt entry, their IRQ handler must be called
+ * vApplicationFPUSafeIRQHandler(), and if the application writer does not want
+ * FPU registers to be saved on interrupt entry their IRQ handler must be
+ * called vApplicationIRQHandler().
+ */
+.align 4
+.weak vApplicationIRQHandler
+.type vApplicationIRQHandler, %function
+vApplicationIRQHandler:
+    PUSH    {LR}
+
+    VMRS    R1, FPSCR
+    VPUSH   {D0-D7}
+    PUSH    {R1}
+
+    BLX     vApplicationFPUSafeIRQHandler
+
+    POP     {R0}
+    VPOP    {D0-D7}
+    VMSR    FPSCR, R0
+
+    POP     {PC}
+
+/*-----------------------------------------------------------*/
+
+.align 4
+.weak vApplicationFPUSafeIRQHandler
+.type vApplicationFPUSafeIRQHandler, %function
+vApplicationFPUSafeIRQHandler:
+    B       vApplicationFPUSafeIRQHandler
+
+/*-----------------------------------------------------------*/
+
 /*
  * UBaseType_t ulPortCountLeadingZeros( UBaseType_t ulBitmap );
  *