KVM: arm64: Convert lazy FPSIMD context switch trap to C

To make the lazy FPSIMD context switch trap code easier to hack on,
this patch converts it to C.

This is not amazingly efficient, but the trap should typically only
be taken once per host context switch.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>