ucode.S: handle negative offsets in load/store instructions
ucode.S: handle negative offsets in load/store instructions

RSP vector load/store instructions have a signed 7-bit offset.
Currently, it was being handled as unsigned offset, so negative
numbers were not allowed. Now, they are correctly handled so it is
possible to write instructions such as "sqv $v00,0, -2,s0" which is
perfectly valid.

DragonMinded/libdragon,sp1187/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,sp1187/libdragon,sp1187/libdragon