rs6000: Adjust -mdejagnu-cpu to filter out -mtune [PR106345]

rs6000: Adjust -mdejagnu-cpu to filter out -mtune [PR106345]

As PR106345 shows, when configuring compiler with an explicit
option --with-tune=<value>, it would cause some test cases to
fail if their test points are sensitive to tune setting, such
as: group_ending_nop, loop align etc.  It doesn't help that
even to specify one explicit -mcpu=.

This patch is to adjust the behavior of -mdejagnu-cpu by
filtering out all -mcpu= and -mtune= options, then test cases
would use <cpu> as tune as the one specified by -mdejagnu-cpu.

2022-07-25  Peter Bergner  <bergner@linux.ibm.com>
	    Kewen Lin  <linkw@linux.ibm.com>

	PR testsuite/106345

gcc/ChangeLog:

	* config/rs6000/rs6000.h (DRIVER_SELF_SPECS): Adjust -mdejagnu-cpu
	to filter out all -mtune options.

(cherry picked from commit 75d20d6c84c12bedd65a904e462f02f0b9eb3f77)