LoongArch: configs: Disable CONFIG_LEGACY_PTYS to fix gnulib test failures

The legacy PTY driver (CONFIG_LEGACY_PTYS) is deprecated and not
required for most modern systems. It causes unexpected behavior
during gnulib tests, leading to test failures related to
pseudo-terminal handling.

Disabling CONFIG_LEGACY_PTYS removes the old /dev/pty* and
/dev/tty* interfaces, allowing gnulib tests to run successfully
with the modern /dev/pts/* subsystem.

Signed-off-by: yangxiaojuan <yangxiaojuan@loongson.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
(cherry picked from commit 026f020)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>

Conflicts:
	arch/loongarch/configs/loongson3_defconfig

Author: AaronDot

arch/loongarch/configs/deepin_loongarch_desktop_defconfig

@@ -5839,3 +5839,4 @@ CONFIG_RV=y
 CONFIG_RV_MON_WWNR=y
 # CONFIG_STRICT_DEVMEM is not set
 # CONFIG_RUNTIME_TESTING_MENU is not set
+# CONFIG_LEGACY_PTYS is not set

arch/loongarch/configs/loongson3_defconfig

@@ -1153,3 +1153,4 @@ CONFIG_SCHEDSTATS=y
 # CONFIG_DEBUG_PREEMPT is not set
 # CONFIG_FTRACE is not set
 CONFIG_UNWINDER_ORC=y
+# CONFIG_LEGACY_PTYS is not set