Makefile: Set CGO_ENABLED=0 for runtimetest

And also set '-installsuffix cgo'.  runtimetest is designed to be run
from inside a container, which may include a mount namespace that has
no linker or dynamic libraries.  These changes give us a static
runtimetest binary [1,2] which will work in those conditions.

[1]: https://golang.org/cmd/cgo/#hdr-Using_cgo_with_the_go_command
[2]: golang/go#9344 (comment)
     Subject: cmd/go: With Go 1.4, cannot statically link a program
       which imports net

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

Makefile

@@ -12,7 +12,7 @@ tool:
 
 .PHONY: runtimetest
 runtimetest:
-	go build -tags "$(BUILDTAGS)" -o runtimetest ./cmd/runtimetest
+	CGO_ENABLED=0 go build -installsuffix cgo -tags "$(BUILDTAGS)" -o runtimetest ./cmd/runtimetest
 
 .PHONY: man
 man: