gdb: LoongArch: Implement the software_single_step gdbarch method

When execute the following command on LoongArch:

  make check-gdb TESTS="gdb.base/branch-to-self.exp"

there exist the following failed testcases:

  FAIL: gdb.base/branch-to-self.exp: single-step: si (timeout)
  FAIL: gdb.base/branch-to-self.exp: break-cond: side=host: continue to breakpoint: continue to break (timeout)
  FAIL: gdb.base/branch-to-self.exp: break-cond: side=host: p counter (timeout)

Implement the software_single_step gdbarch method to decode the current
branch instruction and determine the address of the next instruction on
LoongArch to fix the above failed testcases.

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