LoongArch: gas: Adjust DWARF CIE alignment factors

Set DWARF2_CIE_DATA_ALIGNMENT (data alignment factors) to -8.
It helps to save space.

Data Alignment Factor
A signed LEB128 encoded value that is factored out of all offset
instructions that are associated with this CIE or its FDEs. This value
shall be multiplied by the register offset argument of an offset
instruction to obtain the new offset value.