powerpc: Add a proper syscall for switching endianness

We currently have a "special" syscall for switching endianness. This is
syscall number 0x1ebe, which is handled explicitly in the 64-bit syscall
exception entry.

That has a few problems, firstly the syscall number is outside of the
usual range, which confuses various tools. For example strace doesn't
recognise the syscall at all.

Secondly it's handled explicitly as a special case in the syscall
exception entry, which is complicated enough without it.

As a first step toward removing the special syscall, we need to add a
regular syscall that implements the same functionality.

The logic is simple, it simply toggles the MSR_LE bit in the userspace
MSR. This is the same as the special syscall, with the caveat that the
special syscall clobbers fewer registers.

This version clobbers r9-r12, XER, CTR, and CR0-1,5-7.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/systbl.S

@@ -22,6 +22,7 @@
 #define PPC_SYS(func)		.llong	DOTSYM(ppc_##func),DOTSYM(ppc_##func)
 #define OLDSYS(func)		.llong	DOTSYM(sys_ni_syscall),DOTSYM(sys_ni_syscall)
 #define SYS32ONLY(func)		.llong	DOTSYM(sys_ni_syscall),DOTSYM(compat_sys_##func)
+#define PPC64ONLY(func)		.llong	DOTSYM(ppc_##func),DOTSYM(sys_ni_syscall)
 #define SYSX(f, f3264, f32)	.llong	DOTSYM(f),DOTSYM(f3264)
 #else
 #define SYSCALL(func)		.long	sys_##func
@@ -29,6 +30,7 @@
 #define PPC_SYS(func)		.long	ppc_##func
 #define OLDSYS(func)		.long	sys_##func
 #define SYS32ONLY(func)		.long	sys_##func
+#define PPC64ONLY(func)		.long	sys_ni_syscall
 #define SYSX(f, f3264, f32)	.long	f32
 #endif
 #define SYSCALL_SPU(func)	SYSCALL(func)