lang/go-devel: Add support for aarch64

AArch64 support was recently merged by the Go project and is slated to be in
go1.14. This patch adds freebsd/arm64 support to lang/go-devel first, the
intention is to similarly update lang/go when go1.14 is officially released.

The most notable change is migration to the new bootstrap [1] because aarch64
is not supported by go1.4.

[1] https://github.com/dmgk/go-bootstrap

Reviewed by: tobik, Mikaël Urankar <mikael.urankar@gmail.com>
Approved by: tz (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D22316