Make the Processor Status register match a real 6502 at power-on.

When describing the CPU's reset pin, the W65C02S data sheet says:

> All Registers are initialized by software except the Decimal and Interrupt
> disable mode select bits of the Processor Status Register (P) are initialized
> by hardware.

It then has a diagram of the power-on state of the processor status register:

>     7 6 5 4 3 2 1 0
>     * * 1 1 0 1 * *
>     N V - B D I Z C
>
> * = software initialized

Confusingly the text indicates that only the D and I flags are initialised by
hardware, while the diagram indicates that the B flag is initialised too.

Meanwhile, https://www.nesdev.org/wiki/CPU_power_up_state says that
the power-on state of the NES CPU is $34 (exactly matching the diagram above)
but https://www.nesdev.org/wiki/Status_flags#The_B_flag says that the B flag
does not physically exist within P register, it's only relevant in the copy
of P that gets pushed to the stack by BRK (set), PHP (set), or an interrupt
signal (cleared).

As a result, the most sensible power-on state for the processor status register
is with the "interrupt disable" flag set and everything else cleared.

src/test/java/com/loomcom/symon/CpuImmediateModeTest.java

@@ -29,7 +29,7 @@ public class CpuImmediateModeTest extends TestCase {
         assertEquals(0, cpu.getYRegister());
         assertEquals(0x200, cpu.getProgramCounter());
         assertEquals(0xff, cpu.getStackPointer());
-        assertEquals(0x20, cpu.getProcessorStatus());
+        assertEquals(0x24, cpu.getProcessorStatus());
     }
 
     /*