Add support for a bootstrap binary

Hello,

Currently, on the supported systems, one way to build Go on the host machine is to build it from a binary bootstrap toolchain because the source bootstrap toolchain (Go 1.4) does not target some supported architectures (ppc64le, aarch64...).

In practice, I would like to be able to build Go on a *aarch64* machine. Then, cross-compiled Go packages to run them on an *arm* machine.

References:
- https://golang.org/doc/install/source#bootstrapFromBinaryRelease
- https://golang.org/doc/install/source#environment ($GOHOSTOS and $GOHOSTARCH)

Best regards,
Kevin Vigouroux.