Linker errors with Go 1.12

[aclements]

The assembly-optimized implementations of several of the functions in this package refer to the unexported function runtime.panicindex, which is no longer available in Go 1.12. For example, building the tests fails with:

go get -d -t gorgonia.org/vecf64
go test -tags avx gorgonia.org/vecf64
# gorgonia.org/vecf64.test
gorgonia.org/vecf64.Add: relocation target runtime.panicindex not defined for ABI0 (but is defined for ABIInternal)
gorgonia.org/vecf64.Div: relocation target runtime.panicindex not defined for ABI0 (but is defined for ABIInternal)
gorgonia.org/vecf64.InvSqrt: relocation target runtime.panicindex not defined for ABI0 (but is defined for ABIInternal)
gorgonia.org/vecf64.Mul: relocation target runtime.panicindex not defined for ABI0 (but is defined for ABIInternal)
gorgonia.org/vecf64.Sqrt: relocation target runtime.panicindex not defined for ABI0 (but is defined for ABIInternal)
gorgonia.org/vecf64.Sub: relocation target runtime.panicindex not defined for ABI0 (but is defined for ABIInternal)
FAIL	gorgonia.org/vecf64 [build failed]

The recommended fix for this is to wrap these functions with a Go function that checks their invariants and panics before calling the (unexported) assembly implementation. This Go function will be small enough that it gets inlined, so there's no additional overhead.

See the (still in-progress, but almost committed) design doc here: https://go-review.googlesource.com/c/proposal/+/150077/4/design/27539-internal-abi.md#342