[GOLD] Allow for larger alignment in script_test_15

[Cary Coutant]

> PowerPC64 aligns .got to a 256 byte boundary.  This tends to bump the
> data segment file size.  OK?
>
>         * testsuite/script_test_15a.sh: Allows larger p_filesz.
>         * testsuite/script_test_15b.sh: Likewise.
>         * testsuite/script_test_15c.sh: Likewise.

OK. Sorry, I thought I'd left enough room. Thanks!

-cary