Add system call support and related uaccess.h for LoongArch.

Q: Why keep _ARCH_WANT_SYS_CLONE definition while there is clone3:
A: The latest glibc release has some basic support for clone3 but it is
   not complete. E.g., pthread_create() and spawni() have converted to
   use clone3 but fork() will still use clone. Moreover, some seccomp
   related applications can still not work perfectly with clone3. E.g.,
   Chromium sandbox cannot work at all and there is no solution for it,
   which is more terrible than the fork() story [1].

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2936184Reviewed-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>