powerpc: move ASM_CONST and stringify_in_c() into asm-const.h

This patch moves ASM_CONST() and stringify_in_c() into
dedicated asm-const.h, then cleans all related inclusions.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: asm-compat.h should include asm-const.h]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/copyuser_64.S

@@ -9,6 +9,7 @@
 #include <asm/processor.h>
 #include <asm/ppc_asm.h>
 #include <asm/export.h>
+#include <asm/asm-compat.h>
 
 #ifdef __BIG_ENDIAN__
 #define sLd sld		/* Shift towards low-numbered address. */