allow to switch processes at runtime from big-endian to little-endian mode
(and vice versa) on PowerPC processors.  Since the other architectures
don't support this feature, they currently will just fail and return
-EINVAL.

This patch adds just minimal overhead and implements the PR_GET_ENDIAN
call for all other architectures by returning the native endianess of the
architecture.  Furthermore, calling prctl(PR_SET_ENDIAN) with the native
endianess of the architecture will succeed, while trying to set another
(not-supported) endianess, will fail.

The patch can be tested with the following program:

#include <stdio.h>
#include <linux/prctl.h>

int main(int argc, char **argv)
{
	int endian, ret;

	ret = prctl(PR_GET_ENDIAN, &endian);
	if (ret)
		perror("prctl(PR_GET_ENDIAN) not implemented");
	printf("current process/machine is running in %s endian mode (%d)\n",
		endian == PR_ENDIAN_LITTLE ? "little":"big", endian);

	/* setting native endianess should succeed */
	ret = prctl(PR_SET_ENDIAN, endian);
	printf("prctl(PR_SET_ENDIAN,%d) should succeed: %s\n",
		endian, ret == 0 ? "OK":"FAIL");

	/* setting foreign endianess should fail */
	endian = (endian == PR_ENDIAN_LITTLE) ?
		PR_ENDIAN_BIG : PR_ENDIAN_LITTLE;
	ret = prctl(PR_SET_ENDIAN, endian);
	printf("prctl(PR_SET_ENDIAN,%d) should fail: %s\n",
		endian, ret == 0 ? "OK":"FAIL");
}
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: Anton Blanchard <anton@samba.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>