Missing flags for FreeBSD shared link as used by cgo in default build.

R=rsc
https://golang.org/cl/157065
diff --git a/src/Make.pkg b/src/Make.pkg
index f7b8506..d763951 100644
--- a/src/Make.pkg
+++ b/src/Make.pkg
@@ -103,6 +103,7 @@
 # Have to run gcc with the right size argument on hybrid 32/64 machines.
 _CGO_CFLAGS_386=-m32
 _CGO_CFLAGS_amd64=-m64
+_CGO_LDFLAGS_freebsd=-shared -lpthread -lm
 _CGO_LDFLAGS_linux=-shared -lpthread -lm
 _CGO_LDFLAGS_darwin=-dynamiclib -Wl,-undefined,dynamic_lookup