Add support for freebsd/arm64

[dmgk]

Go 1.14 will add support for freebsd/arm64, golang.org/x/sys@33540a1f6037 is the earliest commit that brings in necessary arm64 bits.

[codecov-io]

Codecov Report

Merging #324 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #324   +/-   ##
=======================================
  Coverage   72.72%   72.72%           
=======================================
  Files          12       12           
  Lines         308      308           
=======================================
  Hits          224      224           
  Misses         76       76           
  Partials        8        8

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[caarlos0]

thanks