gdb: RISC-V: Refine lr/sc sequence support

Per RISC-V spec, the lr/sc sequence can consist of up to 16 instructions, and we
cannot insert breakpoints in the middle of this sequence. Before this, we only
detected a specific pattern (the most common one). This patch improves this part
and now supports more complex pattern detection.

Signed-off-by: Yang Liu <liuyang22@iscas.ac.cn>
Approved-By: Andrew Burgess <aburgess@redhat.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>