testsuite: Adjust powerpc ppc-fortran.exp to support dg-{warning,error}

testsuite: Adjust powerpc ppc-fortran.exp to support dg-{warning,error}

According to Haochen's finding in [1], currently ppc-fortran.exp
doesn't support Fortran specific warning or error messages well.
By looking into it, it's due to that gfortran uses some different
warning/error prefixes as follows:

    set gcc_warning_prefix "\[Ww\]arning:"
    set gcc_error_prefix "(Fatal )?\[Ee\]rror:"

comparing to:

    set gcc_warning_prefix "warning:"
    set gcc_error_prefix "(fatal )?error:"

So this is to override these two prefixes and make it support
dg-{warning,error} checks.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-March/613302.html

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/ppc-fortran/ppc-fortran.exp: Override
	gcc_{warning,error}_prefix with Fortran specific one used in
	gfortran_init.