Support darwin build for kata shim

[raravena80]

Description of problem

Opening up to track building the Kata Shim on macos. Needed to support Kata Containers over xhyve

Expected result

$ cd $GOPATH/src/github.com/kata-containers/shim && make
go build -o kata-shim -ldflags "-X main.version=1.0.0-a69326b63802952b14203ea9c1533d4edb8c1d64"
# Builds the kata-shim exec on darwin

Actual result

$ cd $GOPATH/src/github.com/kata-containers/shim && make
go build -o kata-proxy -ldflags "-X main.version=1.0.0-a69326b63802952b14203ea9c1533d4edb8c1d64"
# github.com/kata-containers/shim
./signals.go:27:2: undefined: syscall.SIGSTKFLT
make: *** [kata-proxy] Error 2

---

[raravena80]

Commenting out the following makes the build go through successfully.

// signals.go
+       //syscall.SIGSTKFLT: true,

However, I believe the right way to implement this is to have a common signals.go file and then have a signals_linux.go and a signals_darwin.go files.

[jodh-intel]

Yes, we could create a signals_darwin.go. All it would need to contain is the darwin-specific handledSignalsMap. Or we could make the file even smaller by requiring that each arch-specific signals file define archSignalsMap [map][syscall.Signal]bool which for darwin might be empty.

But looking again at the man pages, it appears the kernel never sends SIGSTKFLT, so the simplest fix might be to just remove SIGSTKFLT entirely. Is that sufficient to allow a darwin build?

[raravena80]

@jodh-intel yep, that's enough

[raravena80]

@jodh-intel actually there's more. The terminal_linux.go is specific to Linux. I'll submit a PR and we can iterate.