build-sys: fix parallel builds w/setarch links

build-sys: fix parallel builds w/setarch links

The symlink generation tries to write to the sys-utils/ subdir but does
not make sure that dir exists.  This can sometimes lead to parallel build
failures when building out-of-tree like:
...
echo ".so man8/setarch.8" > sys-utils/linux64.8
/bin/bash: sys-utils/linux64.8: No such file or directory
Makefile:11503: recipe for target 'sys-utils/linux64.8' failed
make: *** [sys-utils/linux64.8] Error 1

References: https://bugs.gentoo.org/511812
Signed-off-by: Mike Frysinger <vapier@chromium.org>