fix 1.9.4 cross compile problem

[joyhope]

fix #733

[codecov-io]

Codecov Report

Merging #737 into master will increase coverage by 0.14%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #737      +/-   ##
==========================================
+ Coverage   50.52%   50.67%   +0.14%     
==========================================
  Files          42       41       -1     
  Lines        3119     3104      -15     
==========================================
- Hits         1576     1573       -3     
+ Misses       1405     1387      -18     
- Partials      138      144       +6

Impacted Files	Coverage Δ
gc.go	49% <0%> (-1%)	⬇️
build.go
cmd/gb/build.go	71.34% <0%> (+66.47%)	⬆️

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