build: fix unsafe.Sizeof for llgo:type C

[visualfc]

unsafe.Sizeof 在 types.check 中转换为整数值，所以在 types.check 之前预先分析 []*ast.File 查找 llgo:type C 标志，用来判断 prog.TypesSize.Sizeof 和 prog.gocvt.cvtType 。

• internal/packages
  func (Deduper).SetPreload(fn func(pkg *types.Package, syntax []*ast.File))
func (Deduper).SetPkgPath(fn func(path, name string) string)
• internal/build
  deduper.SetPreload for check llgo:type C before types.check
• ssa
  (Program).SetTypeBackground(fullName string, bg Background)
  use the goTypes.typbg map for types.Sizes and goTypes.cvtType
• internal/llgen
  use internal/packages.Deduper.SetPkgPath for custome pkgPath.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.72%. Comparing base (21a2f71) to head (830c404).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #524   +/-   ##
=======================================
  Coverage   97.71%   97.72%           
=======================================
  Files          19       19           
  Lines        4514     4523    +9     
=======================================
+ Hits         4411     4420    +9     
  Misses         87       87           
  Partials       16       16           

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