Add support for NOP_EXIT_SILENT

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>
Merge pull request #2 from Franck79/63f04c79589c1e8ef682bfa51a38853e3ce1bf1a

Add NOP_EXIT_SILENT support