z/calloc: Enable riscv64 builds

[kraj]

rv64 go port is available in latest go versions starting 1.14+

Signed-off-by: Khem Raj [email protected]

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This change is 

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[kraj]

This is a oneliner build fix, do you need CLA for this? if so, please make change yourself and I Iam totally ok with this.

[cbrake]

any chance this fix can be implemented as it is very simple, and blocking riscv builds for https://github.com/simpleiot

[karlmcguire]

Merged with #264, thanks for the PR guys. Sorry it took so long.

[cbrake]

thanks!