build.plat: add build_{{name}}.json output

[whitequark]

This is useful for custom build runners that don't run a shell; more specifically, WebAssembly based tooling.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 91.24%. Comparing base (f4804c0) to head (0721f60).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
amaranth/build/plat.py	25.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1617      +/-   ##
==========================================
- Coverage   91.24%   91.24%   -0.01%     
==========================================
  Files          44       44              
  Lines       11395    11394       -1     
  Branches     2218     2223       +5     
==========================================
- Hits        10397    10396       -1     
- Misses        833      836       +3     
+ Partials      165      162       -3     

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