This patch consolidates macros used to generate assembly for
compatibility across different CPUs or configs.  A new header,
asm-powerpc/asm-compat.h contains the main compatibility macros.  It
uses some preprocessor magic to make the macros suitable both for use
in .S files, and in inline asm in .c files.  Headers (bitops.h,
uaccess.h, atomic.h, bug.h) which had their own such compatibility
macros are changed to use asm-compat.h.

ppc_asm.h is now for use in .S files *only*, and a #error enforces
that.  As such, we're a lot more careless about namespace pollution
here than in asm-compat.h.

While we're at it, this patch adds a call to the PPC405_ERR77 macro in
futex.h which should have had it already, but didn't.

Built and booted on pSeries, Maple and iSeries (ARCH=powerpc).  Built
for 32-bit powermac (ARCH=powerpc) and Walnut (ARCH=ppc).
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>