Use "host_machine", not "target_machine" for cross-compilation

[kelson42]

Read https://mesonbuild.com/Cross-compilation.html for all the rationals.

[codecov]

Codecov Report

Merging #290 (1d4e32d) into master (321c55c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #290   +/-   ##
=======================================
  Coverage   51.18%   51.18%           
=======================================
  Files          21       21           
  Lines        1940     1940           
  Branches     1152     1150    -2     
=======================================
  Hits          993      993           
+ Misses        945      943    -2     
- Partials        2        4    +2     

Impacted Files	Coverage Δ
src/tools.cpp	86.49% <0.00%> (ø)

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