gdb: LoongArch: Add and use cond_branch_destination_address()

In the current loongarch_deal_with_atomic_sequence(), it is just a loop
through a ll/sc atomic instruction sequence, the instructions before the
condition branch are not actually executed, thus the condition register
value is not proper to determine the destination address.

Add a new function cond_branch_destination_address() to calculate the
destination address of a condition branch instruction under an assumed
true condition, then only put a breakpoint at this address when it is
outside of the ll/sc atomic instruction sequence.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>