ARROW-6966: [Go] Set a default memset for when the platform doesn't set one
ARROW-6966: [Go] Set a default memset for when the platform doesn't set one

Closes #5714 from jsternberg/master and squashes the following commits:

a241cf60d <Jonathan A. Sternberg> ARROW-6966:  Set a default memset for when the platform doesn't set one

Authored-by: Jonathan A. Sternberg <3692bfa45759a67d83aedf0045f6cb635a966abf@influxdata.com>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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