testsuite: Fix pr47614.f test

This test FAILs because
f951: Error: '-mabi=ieeelongdouble' requires full ISA 2.06 support
compiler exited with status 1
FAIL: gfortran.dg/pr47614.f   -O0  (test for excess errors)
As powerpc64le* only supports -mcpu=power8 and newer, I think we shouldn't
be testing with that option.

2022-01-04  Jakub Jelinek  <jakub@redhat.com>

	* gfortran.dg/pr47614.f: Don't use -mcpu=power4 for
	powerpc64le*-*-linux*.