Data abort test doesn't work on ARMv4

[thejpster]

Our data abort exception test relies on an unaligned load triggering a data abort. This does not happen on ARM7TDMI.

A more reliable way to trigger a data abort on that processor is to read a value from an address that is reserved. e.g.

core::arch::naked_asm!(
	"ldr     r0, =0xC0000000",
	"ldr     r0, [r0]",
	"bx      lr",
);

The address 0xC000_0000 is not a valid address on the LPC2138 microcontroller I tested with because it's in the reserved range.