This patch enables the Mips .set directive to define aliases

The .set directive in the Mips the assembler can be
used to set the value of a symbol to an expression.
This changes the symbol's value and type to conform
to the expression's.

Syntax: .set symbol, expression

This patch implements the parsing of the above syntax
and enables the parser to use defined symbols when
parsing operands.

Contributor: Vladimir Medic
Remove some commented out code.