powerpc: Use -mtraceback=no

gcc 4.7 will be more strict about parsing the -mtraceback option:

 gcc: error: unrecognized argument in option '-mtraceback=none'
 gcc: note: valid arguments to '-mtraceback=' are: full no part

gcc used to do a 2 char compare so both "no" and "none" would
match. Switch to using -mtraceback=no should work everywhere.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>