go: update to build on macOS

[peterjgilbert]

This PR factors out some OS-specific syscall parameters for conditional compilation.

This does not change the fact that macOS is not officially supported by Oasis Core.

[codecov]

Codecov Report

Merging #3333 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3333      +/-   ##
==========================================
+ Coverage   65.21%   65.23%   +0.02%     
==========================================
  Files         371      371              
  Lines       33151    33151              
==========================================
+ Hits        21620    21627       +7     
+ Misses       8339     8323      -16     
- Partials     3192     3201       +9     

Impacted Files	Coverage Δ
...o/common/crypto/signature/signers/plugin/plugin.go	57.29% <100.00%> (ø)
go/oasis-node/cmd/common/isatty.go	100.00% <100.00%> (ø)
go/oasis-node/cmd/common/metrics/disk.go	65.38% <0.00%> (-19.24%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	78.94% <0.00%> (-10.53%)	⬇️
go/consensus/tendermint/api/errors.go	90.00% <0.00%> (-10.00%)	⬇️
go/keymanager/client/client.go	81.41% <0.00%> (-4.43%)	⬇️
...n/crypto/signature/signers/memory/memory_signer.go	72.41% <0.00%> (-3.45%)	⬇️
go/worker/common/committee/node.go	74.86% <0.00%> (-2.80%)	⬇️
go/storage/mkvs/lookup.go	72.63% <0.00%> (-2.11%)	⬇️
go/runtime/committee/nodes.go	69.69% <0.00%> (-1.52%)	⬇️
... and 16 more

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