sh: define __BIG_ENDIAN for math-emu

Fix this by defining both ENDIAN macros in
<asm/sfp-machine.h> so that they can be utilized in
<math-emu/soft-fp.h> according to the latter's comment:
/* Allow sfp-machine to have its own byte order definitions. */

(This is what is done in arch/nds32/include/asm/sfp-machine.h.)

This placates these build warnings:

In file included from ../arch/sh/math-emu/math.c:23:
.../include/math-emu/single.h:50:21: warning: "__BIG_ENDIAN" is not defined, evaluates to 0 [-Wundef]
50 | #if __BYTE_ORDER == __BIG_ENDIAN
In file included from ../arch/sh/math-emu/math.c:24:
.../include/math-emu/double.h:59:21: warning: "__BIG_ENDIAN" is not defined, evaluates to 0 [-Wundef]
59 | #if __BYTE_ORDER == __BIG_ENDIAN

Fixes: 4b565680d163 ("sh: math-emu support")
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Yoshinori Sato <[e-mail]>
Cc: John Paul Adrian Glaubitz <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Tested-by: John Paul Adrian Glaubitz <[e-mail]>
Signed-off-by: Rich Felker <[e-mail]>

arch/sh/include/asm/sfp-machine.h

···
 #ifndef _SFP_MACHINE_H
 #define _SFP_MACHINE_H
 
+#ifdef __BIG_ENDIAN__
+#define __BYTE_ORDER __BIG_ENDIAN
+#define __LITTLE_ENDIAN 0
+#else
+#define __BYTE_ORDER __LITTLE_ENDIAN
+#define __BIG_ENDIAN 0
+#endif
+
 #define _FP_W_TYPE_SIZE		32
 #define _FP_W_TYPE		unsigned long
 #define _FP_WS_TYPE		signed long