runtime, go/build: re-enable cgo on FreeBSD.

Fixes #7331.

LGTM=dave, iant
R=golang-codereviews, dave, gobot, iant
CC=golang-codereviews
https://golang.org/cl/89150043

src/pkg/go/build/build.go

@@ -264,6 +264,7 @@ var cgoEnabled = map[string]bool{
 	"dragonfly/amd64": true,
 	"freebsd/386":     true,
 	"freebsd/amd64":   true,
+	"freebsd/arm":     true,
 	"linux/386":       true,
 	"linux/amd64":     true,
 	"linux/arm":       true,

src/pkg/runtime/sys_freebsd_arm.s

@@ -367,6 +367,7 @@ TEXT runtime·casp(SB),NOSPLIT,$0
 TEXT runtime·cas(SB),NOSPLIT,$0
 	B runtime·armcas(SB)
 
+// TODO(minux): this only supports ARMv6K+.
 TEXT runtime·read_tls_fallback(SB),NOSPLIT,$-4
-	MOVW $0xffff1000, R0
-	MOVW (R0), R0
+	WORD $0xee1d0f70 // mrc p15, 0, r0, c13, c0, 3
+	RET