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