gdb: LoongArch: Emulate floating-point branch instructions

Add bceqz and bcnez cases in loongarch_insn_is_cond_branch() and
loongarch_next_pc() to emulate floating-point branch instructions.

Here are the references:

https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#_bceqz_bcnez
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#table-table-of-instruction-encoding

Approved-by: Kevin Buettner <kevinb@redhat.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>