[MERGE #4247 @sigatrev] ARM64: use uint16 for char codes
[MERGE #4247 @sigatrev] ARM64: use uint16 for char codes

Merge pull request #4247 from sigatrev:charcode

these should not be sign-extended when loaded into registers.

Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore