um: stop polluting the namespace with registers.h contents

Only one extern in there is needed in processor-generic.h, and it's
not needed anywhere else.  So move it over there and get rid of
the include in processor-generic.h, adding includes of registers.h
to the few files that need the declarations in it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/process.c

@@ -31,6 +31,7 @@
 #include <kern_util.h>
 #include <os.h>
 #include <skas.h>
+#include <registers.h>
 #include <linux/time-internal.h>
 
 /*