ANDROID: KVM: arm64: Prevent HVC calls outside of the core kernel text

Modules can easily wreak havoc in the hypervisor by calling into
it randomly, making it very hard to understand what is going on.

Given that limiting hypercalls to the core kernel is actually
pretty easy (a simple comparaison with _text and _etext), let's
implement that.

This is made extra-complicated due to KASLR and the disjointed
VA spaces (you can't just refer to _text, as this results in a
relative reference...).

Bug: 210011561
Signed-off-by: Marc Zyngier <maz@kernel.org>
Change-Id: I2f21871d7fe0fb22fd3660dbc1317ec8968d5b61
Signed-off-by: Sebastian Ene <sebastianene@google.com>