gdb: LoongArch: Add show-debug-regs maintenance command

This patch register the command "maint set show-debug-regs on/off"
and make it settable by the user. If show-debug-regs is enabled,
the debug register values are shown when GDB inserts or removes a
hardware breakpoint or watchpoint. This is helpful for the use and
development of hardware watchpoints.

With this patch, the effect of this maintenance command as follows:

lihui@bogon:~$ cat test.c
int a = 0;
int main()
{
	a = 1;
	return 0;
}
lihui@bogon:~$ gcc -g test.c -o test
lihui@bogon:~$ gdb test
...
(gdb) watch a
Hardware watchpoint 1: a
(gdb) maint set show-debug-regs on
(gdb) r
Starting program: /home/lihui/test
...
...

prepare_to_resume thread 41525
...
insert_watchpoint (addr=0x12000803c, len=4, type=hw-write-watchpoint):
	BREAKPOINTs:
	BP0: addr=0x0, ctrl=0x00000000, ref.count=0
	BP1: addr=0x0, ctrl=0x00000000, ref.count=0
	BP2: addr=0x0, ctrl=0x00000000, ref.count=0
	BP3: addr=0x0, ctrl=0x00000000, ref.count=0
	BP4: addr=0x0, ctrl=0x00000000, ref.count=0
	BP5: addr=0x0, ctrl=0x00000000, ref.count=0
	BP6: addr=0x0, ctrl=0x00000000, ref.count=0
	BP7: addr=0x0, ctrl=0x00000000, ref.count=0
	WATCHPOINTs:
	WP0: addr=0x0, ctrl=0x00000000, ref.count=0
	WP1: addr=0x0, ctrl=0x00000000, ref.count=0
	WP2: addr=0x0, ctrl=0x00000000, ref.count=0
	WP3: addr=0x0, ctrl=0x00000000, ref.count=0
	WP4: addr=0x0, ctrl=0x00000000, ref.count=0
	WP5: addr=0x0, ctrl=0x00000000, ref.count=0
	WP6: addr=0x0, ctrl=0x00000000, ref.count=0
	WP7: addr=0x12000803c, ctrl=0x00000610, ref.count=1
...
remove_watchpoint (addr=0x12000803c, len=4, type=hw-write-watchpoint):
	BREAKPOINTs:
	BP0: addr=0x0, ctrl=0x00000000, ref.count=0
	BP1: addr=0x0, ctrl=0x00000000, ref.count=0
	BP2: addr=0x0, ctrl=0x00000000, ref.count=0
	BP3: addr=0x0, ctrl=0x00000000, ref.count=0
	BP4: addr=0x0, ctrl=0x00000000, ref.count=0
	BP5: addr=0x0, ctrl=0x00000000, ref.count=0
	BP6: addr=0x0, ctrl=0x00000000, ref.count=0
	BP7: addr=0x0, ctrl=0x00000000, ref.count=0
	WATCHPOINTs:
	WP0: addr=0x0, ctrl=0x00000000, ref.count=0
	WP1: addr=0x0, ctrl=0x00000000, ref.count=0
	WP2: addr=0x0, ctrl=0x00000000, ref.count=0
	WP3: addr=0x0, ctrl=0x00000000, ref.count=0
	WP4: addr=0x0, ctrl=0x00000000, ref.count=0
	WP5: addr=0x0, ctrl=0x00000000, ref.count=0
	WP6: addr=0x0, ctrl=0x00000000, ref.count=0
	WP7: addr=0x0, ctrl=0x00000000, ref.count=0

Hardware watchpoint 1: a

Old value = 0
New value = 1
main () at test.c:5
5		return 0;
(gdb)

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