relaxable mov rd,#immx

Currently, this:

``` asm
.text
_start:
    mov r0, foo

.data
.align 3
foo:
.word 0x12345678
```

...fails with

```
(.text+0x0): relocation truncated to fit: R_EPIPHANY_16 against `no symbol'
```

It would be nice to have a relaxable mov that expanded (if needed) to:

``` asm
    mov r0, %low(foo)
    movt ro,%high(foo)
```
