MIPS: Wire up clone3 syscall

Wire up the new clone3 syscall for MIPS, using save_static_function() to
generate a wrapper that saves registers $s0-$s7 prior to invoking the
generic sys_clone3 function just like we do for plain old clone.

Tested atop 64r6el_defconfig using o32, n32 & n64 builds of the simple
test program from:

  https://lore.kernel.org/lkml/20190716130631.tohj4ub54md25dys@brauner.io/Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Christian Brauner <christian@brauner.io>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: linux-mips@vger.kernel.org

arch/mips/include/asm/unistd.h

@@ -52,6 +52,7 @@
 # endif
 #define __ARCH_WANT_SYS_FORK
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_SYS_CLONE3
 
 /* whitelists for checksyscalls */
 #define __IGNORE_fadvise64_64

arch/mips/kernel/syscall.c

@@ -80,6 +80,7 @@ SYSCALL_DEFINE6(mips_mmap2, unsigned long, addr, unsigned long, len,
 
 save_static_function(sys_fork);
 save_static_function(sys_clone);
+save_static_function(sys_clone3);
 
 SYSCALL_DEFINE1(set_thread_area, unsigned long, addr)
 {

arch/mips/kernel/syscalls/syscall_n32.tbl

@@ -373,4 +373,4 @@
 432	n32	fsmount				sys_fsmount
 433	n32	fspick				sys_fspick
 434	n32	pidfd_open			sys_pidfd_open
-# 435 reserved for clone3
+435	n32	clone3				__sys_clone3

arch/mips/kernel/syscalls/syscall_n64.tbl

@@ -349,4 +349,4 @@
 432	n64	fsmount				sys_fsmount
 433	n64	fspick				sys_fspick
 434	n64	pidfd_open			sys_pidfd_open
-# 435 reserved for clone3
+435	n64	clone3				__sys_clone3

arch/mips/kernel/syscalls/syscall_o32.tbl

@@ -422,4 +422,4 @@
 432	o32	fsmount				sys_fsmount
 433	o32	fspick				sys_fspick
 434	o32	pidfd_open			sys_pidfd_open
-# 435 reserved for clone3
+435	o32	clone3				__sys_clone3