printer: constraints formatting

[mmcloughlin]

Uses buildtag.Format to format constraints in the assembly and stub file
printers. This will ensure // + build and //go:build syntax are used
consistent with the current Go version.

Updates #183

[codecov-commenter]

Codecov Report

Merging #209 (2f15d6a) into master (55d98cc) will decrease coverage by 0.00%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   75.64%   75.64%   -0.01%     
==========================================
  Files          57       57              
  Lines       35114    35120       +6     
==========================================
+ Hits        26563    26567       +4     
- Misses       8399     8400       +1     
- Partials      152      153       +1     

Flag	Coverage Δ
integration	8.78% <20.00%> (+0.04%)	⬆️
unittests	73.30% <40.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
printer/goasm.go	94.01% <40.00%> (-2.48%)	⬇️
printer/stubs.go	78.57% <40.00%> (+6.57%)	⬆️

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