Fix a problem where multigcc.pl sometimes produces lines like "sh-elf-gcc: no input...

Fix a problem where multigcc.pl sometimes produces lines like "sh-elf-gcc: no input files", especially on systems with many cores and for builds with (relatively) few files.  This happened because the slice size was rounded up, which meant that in some cases there werere leftover slices.
Also remove the now unnecessary cores>files checking.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27196 a1c6a512-1295-4272-9138-f99709370657