Adjust 26 vs. 32 bit mode checks
As noted in https://www.riscosopen.org/forum/forums/9/topics/15359, MRS instructions which are executed in user mode may return unknown values for the CPSR E, A, I, F and M fields on ARMv8 CPUs. At the moment the only observed deviation from normal behaviour is that CPUs which lack AArch32 privileged-mode support may return zero for the fields. This was found to confuse some 26 vs. 32 bit mode checks in CLib.
Clearly we're a long way away from having a version of RISC OS which can run on a CPU that lacks AArch32 privileged mode support, but we can still try and make sure user-mode applications are compatible with them. So this change adjusts the mode checks in the stubs and overlay manager so that programs linked with them should function correctly (TEQ is now used instead of MRS)
Note that once we do start porting RISC OS to one of the troublesome CPUs, there are many more potentially troublesome MRS's within the module code that will need adjusting, especially if CPUs start returning values other than zero.