rs6000: Add -mdejagnu-cpu=

rs6000: Add -mdejagnu-cpu=

This adds an option -mdejagnu-cpu=.  This option simply overrides what
is given in -mcpu=.  The reason for this is that with older versions
of DejaGnu the value given in the RUNTESTFLAGS will override the value
a testcase wants to have.

	* config/rs6000/rs6000.c (rs6000_option_override_internal): If
	rs6000_dejagnu_cpu_index is set, use that to override rs6000_cpu_index.
	* config/rs6000/rs6000.opt (mdejagnu-cpu=): New option.

From-SVN: r269322