gdbserver: Add LoongArch/Linux support

gdbserver: Add LoongArch/Linux support

Implement LoongArch/Linux support, including XML target description
handling based on features determined, GPR regset support, and software
breakpoint handling.

In the Linux kernel code of LoongArch, ptrace implements PTRACE_POKEUSR
and PTRACE_PEEKUSR in the arch_ptrace function, so srv_linux_usrregs is
set to yes.

With this patch on LoongArch:

  $ make check-gdb TESTS="gdb.server/server-connect.exp"
  [...]
  # of expected passes		18
  [...]

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