{make,set,swap}context broken on powerpc32

[Paul Mackerras]

I wrote:

> I just wrote a little test program to call sys_swapcontext directly
> and check what happens to r2, and it appears to be doing the right
> thing (i.e. sys_swapcontext with non-NULL second argument is not
> modifying r2).  This is with a 2.6.18-rc7 ppc64 kernel, and a 32-bit
> user process.  I'll try a ppc32 kernel shortly.

And indeed with a 2.6.18 ppc32 kernel the system call is doing the
right thing also.  Also, a similar test with a 64-bit process on a
64-bit kernel confirmed that the 64-bit sys_swapcontext is not
altering r13.

Paul.