rs6000/test: Make ppc-fortran.exp only available for PowerPC target

rs6000/test: Make ppc-fortran.exp only available for PowerPC target

When testing one patch which adds a fortran test case into
test bucket powerpc/ppc-fortran/, I found one unexpected
failure on a non-PowerPC target.  It's due to that
ppc-fortran.exp does not exit early if the testing target
isn't a PowerPC target.  This patch is to make it exit
immediately if the testing target isn't a PowerPC target.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/ppc-fortran/ppc-fortran.exp: Exit immediately if
	the testing target isn't a PowerPC target.