update to ri5cy core interrupt system and toolchain

[lightning_fingers]

in "main()" {
    set_csr (0x300, 0x00000009);            //enable interrupts?
    write_csr(0x305, my_isr);               // isr address?
   
    // big long delay
    for (int i=0;i<10000;i++) asm("NOP");

}

void __attribute__ ((interrupt())) my_isr (){
    *simresult = (unsigned short int) 0xF111;    // flag result
}

which gives me:
00000300 <my_isr>:
 300: ff010113           addi sp,sp,-16
 304: 00f12423           sw a5,8(sp)
 308: 00e12623           sw a4,12(sp)
 30c: 40002703           lw a4,1024(zero) # 400 <simresult>
 310: 0000f7b7           lui a5,0xf
 314: 11178793           addi a5,a5,273 # f111 <_end+0xece5>
 318: 00f72023           sw a5,0(a4)
 31c: 00c12703           lw a4,12(sp)
 320: 00812783           lw a5,8(sp)
 324: 01010113           addi sp,sp,16
 328: 10000073           eret

The compiler is inserting the correct isr handling code......
But when simulated with the latest ri5cy core I get:
#  - DGB - write 0 into CTRL
#        2465870000 ps: Illegal instruction (core 0) at PC 0x00000328:
#        2469762000 ps: Illegal instruction (core 0) at PC 0x00000328:

I've managed work around this version of the tool chain I am using by hacking the memory image that I read into my memory models in my RTL stimulus env.

i.e. replace any opcodes eret (32'h10000073) with mret (32'h30200073)

this is obviously a very short term fix, but does at least allow me to continue this particular RTL simulation.
With this hack, the simulation executes as expected.