need to be able to build a mkfs mach-o

[eyberg]

there is no expectation that we need to cross-compile nanos for darwin anytime soon, however, nvm will be used on osx by january 19th utilizing pre-made packages and we need a mach-o mkfs by then

porting mkfs to go is prob. non-trivial right now && from irc it appears that its just an issue of fighting some clang build errors

on mojave 10.14.2 I have:

➜  ~  /usr/bin/cc --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin18.2.0

[eyberg]

#271