microblaze: kill incorrect __bad_xchg definition

The whole point of the __bad_xchg declaration in
system.h is to give a linker error when a variable
of invalid size is passed to __xchg. The out
of line definition in traps.c defeats this purpose
and does not any value, so remove it here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <monstr@monstr.eu>