KVM: arm64: Add build-time check for duplicate DECLARE_REG use

KVM: arm64: Add build-time check for duplicate DECLARE_REG use

The DECLARE_REG() macro provides a convenient way to create a local
variable initialized from a cpu context in the hyp trap handlers.
However, a common error is to use the macro multiple times in the same
scope with the same register index, but for different logical purposes.

This results in valid C code that compiles without error, but introduces
subtle bugs where a developer expects two different variables to hold
values from two different registers, when in fact they are both sourced
from the same one.

To prevent this entire class of bugs, modify the DECLARE_REG() macro
to declare a dummy variable whose name is derived from the register
index. If the macro is used again with the same index in the same
scope, the compiler will fail with a "redeclaration of variable"
error, turning a subtle runtime bug into an obvious build-time failure.

Signed-off-by: Fuad Tabba <tabba@google.com>
Tested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>