arm64/arm64gen: get system register readable and writeable attribute
arm64/arm64gen: get system register readable and writeable attribute

This patch gets the readable and writeable property of system register from the
generator sysreggen.go and writes it to the sysRegEnc.go file. This attribute
would be used by assembler to check the correctness of system register's
read/write access for MRS/MSR instruction.

Change-Id: I637f86710c9f53e9e621d1dd6b7c4142f7bd4c11
Reviewed-on: https://go-review.googlesource.com/c/arch/+/194937
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/arch,golang/arch