[Sparc] Support relocatable expressions in the assembler

Allow assembler expressions to start with an identifier. This allows for expressions such as
```
b symbol + 4
```
and
```
mov symEnd - symStart, %g1
```

The patch builds upon https://reviews.llvm.org/D47136.

Reviewed By: joerg

Differential Revision: https://reviews.llvm.org/D47458