Make the agent support ppc64le

[waltforme]

When building for ppc64le, I saw error messages like these:

/root/go/pkg/mod/github.com/headzoo/surf@v1.0.1/agent/agent_linux.go:19:23: cannot use buf.Sysname (variable of type [65]uint8) as [65]int8 value in argument to charsToString
/root/go/pkg/mod/github.com/headzoo/surf@v1.0.1/agent/agent_linux.go:29:23: cannot use buf.Release (variable of type [65]uint8) as [65]int8 value in argument to charsToString

This PR adjusts/adds the go build tags so that the agent additionally covers ppc64le.
Legacy format of go build tags are used here just to be consistent with existing ones.