arch-arm: Use ThreadContext in ArmISA::currEL implementation

This is partly reverting a previous patch [1] which was
moving most functionalities within the ISA class.
This evidently does not work well with thread context implementations
which are bypassed by the ISA objects as noted by [2]

[1]: https://gem5-review.googlesource.com/c/public/gem5/+/53624
[2]: https://gem5-review.googlesource.com/c/public/gem5/+/64653

Change-Id: I0c91c76f690542219ffbbf53359531d9dea9e86d
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64914
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/arch/arm/utility.cc

@@ -123,8 +123,8 @@ inAArch64(ThreadContext *tc)
 ExceptionLevel
 currEL(const ThreadContext *tc)
 {
-    return static_cast<ArmISA::ISA *>(
-        const_cast<ThreadContext *>(tc)->getIsaPtr())->currEL();
+    CPSR cpsr = tc->readMiscRegNoEffect(MISCREG_CPSR);
+    return opModeToEL((OperatingMode)(uint8_t)cpsr.mode);
 }
 
 bool