While we at it, why not add a target for Windows as well.

This also introduces a VERBOSE option that defaults to -v, but it empty
when releases so that you can actually see what you're building.

Move an @echo out of shell snippet into the Makefile, as that errored
with @echo: command not found.

Sample run and resulting artifacts:

~~~
% make -f Makefile.release build
% find build -type f -exec file {} \;
build/windows/amd64/coredns: PE32+ executable (console) x86-64 (stripped to external PDB), for MS Windows
build/darwin/amd64/coredns: Mach-O 64-bit x86_64 executable, flags:<NOUNDEFS>
build/linux/ppc64le/coredns: ELF 64-bit LSB executable, 64-bit PowerPC or cisco 7500, version 1 (SYSV), statically linked, stripped
build/linux/amd64/coredns: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped
build/linux/arm/coredns: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), statically linked, stripped
build/linux/s390x/coredns: ELF 64-bit MSB executable, IBM S/390, version 1 (SYSV), statically linked, stripped
build/linux/arm64/coredns: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, stripped

% make -f Makefile.release tar
% find release -type f | sort
release/coredns_0.9.9_darwin_amd64.tgz
release/coredns_0.9.9_linux_amd64.tgz
release/coredns_0.9.9_linux_arm64.tgz
release/coredns_0.9.9_linux_arm.tgz
release/coredns_0.9.9_linux_ppc64le.tgz
release/coredns_0.9.9_linux_s390x.tgz
release/coredns_0.9.9_windows_amd64.tgz
~~~