Perform switch_to_el2 operation in arm_vcpu, totally remove el2 related codes from arceos

[hky1999]

ref pr #31

[hky1999]

do not want any modification in ArceOS's platform.

some mechanism can refer to Jailhouse, something related to switch from ArceOS at el1 into arm_vcpu at el2, after that each exception should be caught by arm_vcpu and dispatch to ArceOS of needed