xen/arm64: Make show_registers working even if current is not correctly set

xen/arm64: Make show_registers working even if current is not correctly set

The commit 8ab802c "xen/arm: Restricted access to IFSR32_EL2 and FPEXC32_EL2"
introduced the use of the variable current in show_registers.

The variable is not correctly set until Xen has finished to boot.
Therefore any call to show_registers will result to crash Xen.

As the register IFSR32_EL2 is only printed when the registers represent
a guest view, Xen can read the value only for this case.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>