Save changes to the RISC-V port layer before making changes necessary to support...

Save changes to the RISC-V port layer before making changes necessary to support pulpino too:
+ Switch positions of the asm functions used to start the kernel and handle traps to enable one to reference to the other.
+ Handle external interrupts (working with Renode emulator).
+ The _sp linker variable is now called __freertos_irq_stack_top.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2603 1d2547de-c912-0410-9cb9-b8ca96c0e9e2