arm: precise float64 software floating point

Adds softfloat64 to generic runtime
(will be discarded by linker when unused)
and adds test for it.  I used the test to check
the software code against amd64 hardware
and then check the software code against
the arm and its simulation of hardware.
The latter should have been a no-op (testing
against itself) but turned up a bug in 5c causing
the vlrt.c routines to miscompile.

These changes make the cmath, math,
and strconv tests pass without any special
accommodations for arm.

R=ken2
CC=golang-dev
https://golang.org/cl/2713042
diff --git a/src/pkg/runtime/export_test.go b/src/pkg/runtime/export_test.go
new file mode 100644
index 0000000..58631c7
--- /dev/null
+++ b/src/pkg/runtime/export_test.go
@@ -0,0 +1,17 @@
+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Export guts for testing.
+
+package runtime
+
+var Fadd64 = fadd64
+var Fsub64 = fsub64
+var Fmul64 = fmul64
+var Fdiv64 = fdiv64
+var F64to32 = f64to32
+var F32to64 = f32to64
+var Fcmp64 = fcmp64
+var Fintto64 = fintto64
+var F64toint = f64toint