Support static builds on ARM

[danxmoran]

We're testing a static build of InfluxDB OSS v2 on ARM, and seeing failures from pkg-config. One warning I saw in the build output was:

Unable to determine cargo target. Using the default.	{"target": "linux_arm64_static"}

This is because the logic for determining a cargo target includes cases for ARM, but only for non-static builds:

pkg-config/libs/flux/build.go

Lines 45 to 67 in b223740

	func (t Target) DetermineCargoTarget(logger *zap.Logger) string {
	switch {
	case t.OS == "linux" && t.Arch == "amd64" && t.Static:
	return "x86_64-unknown-linux-musl"
	case t.OS == "linux" && t.Arch == "amd64" && !t.Static:
	return "x86_64-unknown-linux-gnu"
	case t.OS == "linux" && t.Arch == "386" && !t.Static:
	return "i686-unknown-linux-gnu"
	case t.OS == "linux" && t.Arch == "arm" && t.Arm == "6" && !t.Static:
	return "arm-unknown-linux-gnueabihf"
	case t.OS == "linux" && t.Arch == "arm" && t.Arm == "7" && !t.Static:
	return "armv7-unknown-linux-gnueabihf"
	case t.OS == "linux" && t.Arch == "arm64" && !t.Static:
	return "aarch64-unknown-linux-gnu"
	case t.OS == "darwin" && t.Arch == "amd64":
	return "x86_64-apple-darwin"
	case t.OS == "windows" && t.Arch == "amd64":
	return "x86_64-pc-windows-gnu"
	default:
	logger.Warn("Unable to determine cargo target. Using the default.", zap.String("target", t.String()))
	return ""
	}
	}

.

We should add cases for static ARM builds. My best guesses for what those target should be are:

• ARM64: aarch64-unknown-linux-musl
• ARM 6: arm-unknown-linux-musleabihf
• ARM 7: armv7-unknown-linux-musleabihf