Mips specific standalone assembler addressing mode %hi and %lo.

The constructs %hi() and %lo() represent the high and low 16 
bits of the address. 
Because the 16 bit offset field of an LW instruction is 
interpreted as signed, if bit 15 of the low part is 1 then the 
low part will act as a negative and 1 needs to be added to the 
high part.

Contributer: Vladimir Medic
llvm-svn: 175707