LoongArch: Support referring to FCSRs as $fcsrX

LoongArch: Support referring to FCSRs as $fcsrX

Previously, FCSRs were referred to as $rX, which seemed strange.
We refer to FCSRs as $fcsrX, which ensures compatibility with LLVM
IAS as well.

gas/ChangeLog:

        * config/tc-loongarch.c:
        (loongarch_fc_normal_name): New definition.
        (loongarch_fc_numeric_name): New definition.
        (loongarch_single_float_opcodes): Modify `movgr2fcsr` and
        `movfcsr2gr`.
        testsuite/gas/loongarch/float_op.d: Likewise.
        testsuite/gas/loongarch/float_op.s: Likewise.

include/ChangeLog:

        * opcode/loongarch.h:
        (loongarch_fc_normal_name): New extern.
        (loongarch_fc_numeric_name): New extern.

opcodes/ChangeLog:

        * opcodes/loongarch-dis.c (loongarch_after_parse_args): Support
        referring to FCSRs as $fcsrX.
        * opcodes/loongarch-opc.c (loongarch_args_parser_can_match_arg_helper):
        Likewise.

Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>