buildsystems: Make sure debug files are not executable

Remove executable permissions.
buildsystems: Strip debug files before moving files to the devel artifact

This way we strip symbols from development files as well.
buildsystems: Don't strip symbolic links

Ignore symbolic links, if the source path is in one of the target
directories it will be stripped eventually.
buildsystems: Don't use custom run_sync(), log() and fatal()

When this code was in ostree-build-compile-one it made sense to use
custom version of these functions because there was no pyostbuild
installed in the build chroot, but now we copy it so we can use our
own library functions.
buildsystems: Fix typo

Ops, it's objcopy not objdump.
buildsystems: Only strip ELF executables

We scan several directories which might contain shell scripts and
other non-ELF executables causing objcopy to bail out.
Check the MIME type to detect ELF executables and ignore everything
else.