LoongArch: gas: add support using constant variable in instructions.

LoongArch: gas: add support using constant variable in instructions.

        Instructions that can load immediate support using constant
        variable like ".equ var, 123    li.w/d resgister, var".

gas/
        * config/loongarch-parse.y
        * config/tc-loongarch.c

        Add four testcases.One is a program using constant variable,
        one test using label is unsupported, and another two test
        almost instructions that can load immediate.

gas/
        * testsuite/gas/loongarch/li.d
        * testsuite/gas/loongarch/li.s
        * testsuite/gas/loongarch/imm_ins_label-fail.d
        * testsuite/gas/loongarch/imm_ins_label-fail.l
        * testsuite/gas/loongarch/imm_ins_label-fail.s
        * testsuite/gas/loongarch/imm_ins.d
        * testsuite/gas/loongarch/imm_ins.s
        * testsuite/gas/loongarch/imm_ins_32.d
        * testsuite/gas/loongarch/imm_ins_32.s