testsuite: Ignore warning for unsupported option

The -w option was used in gcc.dg/20020206-1.c to ignore warnings if the
'-fprefetch-loop-arrays' option is not supported by target.

When commit r14-5380-g5c432b0efab54e removed the -w option, some targets
(arm-none-eabi, pru and possibly others) started failing the test:

  cc1: warning: '-fprefetch-loop-arrays' not supported for this target
  FAIL: gcc.dg/20020206-1.c (test for excess errors)

Fix by instructing DejaGnu to prune the '-fprefetch-loop-arrays'
warning.

gcc/testsuite/ChangeLog:

	* gcc.dg/20020206-1.c: Prune warning that
	-fprefetch-loop-arrays is not supported.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>