Add NOP_EXIT_SILENT support

This patch adds support for NOP_EXIT_SILENT (0x000c)
opcode. This opcode acts like NOP_EXIT but doesn't print
anything while exiting.

NOP_EXIT_SILENT was introduce in the new or1k toolchain.

See:
http://www.mail-archive.com/openrisc@lists.openrisc.net/msg00637.html

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>