cmd/internal/obj/arm64: add error checking for system registers.
This CL adds system register error checking test cases. There're two kinds of
error test cases:
1. illegal combination.
MRS should be used in this way: MRS <system register>, <general register>.
MSR should be used in this way: MSR <general register>, <system register>.
Error usage examples:
MRS R8, VTCR_EL2 // ERROR "illegal combination"
MSR VTCR_EL2, R8 // ERROR "illegal combination"
2. illegal read or write access.
Error usage examples:
MSR R7, MIDR_EL1 // ERROR "expected writable system register or pstate"
MRS OSLAR_EL1, R3 // ERROR "expected readable system register"
This CL reads system registers readable and writeable property to check whether
they're used with legal read or write access. This property is named AccessFlags
in sysRegEnc.go, and it is automatically generated by modifing the system register
generator.