* release: add more build targets

This adds amd64, ppc and s390. Rework some other builds/directories.

This builds:
% find build
build
build/darwin
build/darwin/x86_64
build/darwin/x86_64/coredns
build/linux
build/linux/ppc64
build/linux/ppc64/coredns
build/linux/x86_64
build/linux/x86_64/coredns
build/linux/arm64
build/linux/arm64/coredns
build/linux/s390
build/linux/s390/coredns
build/linux/arm
build/linux/arm/coredns

% make -f Makefile.release tar
rm -rf release && mkdir release
tar -zcf release/coredns_0.9.9_linux_x86_64.tgz -C build/linux/x86_64 coredns
tar -zcf release/coredns_0.9.9_darwin_x86_64.tgz -C build/darwin/x86_64 coredns
tar -zcf release/coredns_0.9.9_linux_armv6l.tgz -C build/linux/arm coredns
tar -zcf release/coredns_0.9.9_linux_armv8l.tgz -C build/linux/arm64 coredns
tar -zcf release/coredns_0.9.9_linux_ppc64le.tgz -C build/linux/ppc64 coredns
tar -zcf release/coredns_0.9.9_linux_s390x.tgz -C build/linux/s390 coredns

Checking:
% for i in $(find build -type f); do file $i; done
build/darwin/x86_64/coredns: Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS>
build/linux/ppc64/coredns: ELF 64-bit LSB executable, 64-bit PowerPC or cisco 7500, version 1 (SYSV), statically linked, stripped
build/linux/x86_64/coredns: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped
build/linux/arm64/coredns: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, stripped
build/linux/s390/coredns: ELF 64-bit MSB executable, IBM S/390, version 1 (SYSV), statically linked, stripped
build/linux/arm/coredns: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, stripped

* code review