cmd/go: report trimpath erasing ldflags, and allow override

[xnox]

Add a new boolean option -trimldflags. Only meaningful when -trimpath
is true. Defaults to true for backwards compatibility. Otheriwise when
set to false reports ldflags in buildinfo, in spite of -trimpath
setting. Also when ldflags are trimmed from the output, leave a
reproducible marker that it happened.

Building with '-trimpath -ldflags="-X main.Version=234"' will now emit:
build -trimldflags=true

Adding -trimldflags=false to the above will emit ldflags:
build -ldflags="-X main.Version=234"

Fixes: #63432

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Patch Set 1:

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Message from Dimitri John Ledkov:

Patch Set 2:

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