Use the extended movx instruction instead of mov (#676)

Use the extended movx instruction instead of mov (#676)

The following is from the MSP430X instruction set -

```
MOVX.W Move source word to destination word.

The source operand is copied to the destination. The source operand is
not affected. Both operands may be located in the full address space.
```

The movx instruction allows both the operands to be located in the full
address space and therefore, works with large data model as well.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>