Add support for official MIPS64EL arch on Linux

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This arch is available with official Electron releases starting with Electron 1.8.2-beta.5.

This is a breaking change, since the behavior of the all option changes.

[codecov]

Codecov Report

Merging #800 into master will increase coverage by 5.05%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #800      +/-   ##
========================================
+ Coverage   94.94%   100%   +5.05%     
========================================
  Files          11     11              
  Lines         574    579       +5     
========================================
+ Hits          545    579      +34     
+ Misses         29      0      -29

Impacted Files	Coverage Δ
targets.js	100% <100%> (ø)	⬆️
ignore.js	100% <0%> (+2.63%)	⬆️
mac.js	100% <0%> (+23.72%)	⬆️

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