[POWERPC] Use __always_inline for xchg* and cmpxchg*

This changes the definitions of the xchg and cmpxchg families of
functions in include/asm-powerpc/system.h to be marked __always_inline
rather than __inline__.  The reason for doing this is that we rely on
the compiler inlining them in order to eliminate the references to
__xchg_called_with_bad_pointer and __cmpxchg_called_with_bad_pointer,
which are deliberately left undefined.  Thus this change will enable
us to make the inline keyword be just a hint rather than a directive.

Signed-off-by: Paul Mackerras <paulus@samba.org>