util: implement GetTotalMemory for mac and windows

[algorandskiy]

Summary

Nightly macos build failed with

util/util.go:82:18: undefined: syscall.Sysinfo_t
util/util.go:83:18: undefined: syscall.Sysinfo

Implemented GetTotalMemory for macos and windows.

Test Plan

Added a unit test.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.70%. Comparing base (cbfa226) to head (164fca1).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
util/util_linux.go	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6272   +/-   ##
=======================================
  Coverage   51.69%   51.70%           
=======================================
  Files         646      647    +1     
  Lines       86860    86858    -2     
=======================================
+ Hits        44901    44906    +5     
+ Misses      39095    39084   -11     
- Partials     2864     2868    +4     

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