cmd/gomobile: add optional -macosversion build flag for macOS

[wlynxg]

Add a -macosversion build flag for macOS targets and
pass -mmacosx-version-min to clang when the flag is set.

Leave the flag unset by default so existing macOS builds keep their
current deployment target behavior and avoid backward compatibility
regressions.

Apple documents -mmacosx-version-min here:
https://developer.apple.com/library/archive/documentation/Porting/Conceptual/PortingUnix/compiling/compiling.html

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Hajime Hoshi:

Patch Set 1:

(1 comment)

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Message from wei liu:

Patch Set 2:

(2 comments)

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Message from Hajime Hoshi:

Patch Set 2: Auto-Submit+1 Code-Review+2 Commit-Queue+1

(1 comment)

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2026-05-29T03:00:08Z","revision":"8a635275e408e48c7a43a713b6ec1e6bbdd398b5"}

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Message from Hajime Hoshi:

Patch Set 2: -Commit-Queue

(Performed by <GERRIT_ACCOUNT_60063> on behalf of <GERRIT_ACCOUNT_7938>)

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Patch Set 2:

This CL has passed the run

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Message from golang-scoped@luci-project-accounts.iam.gserviceaccount.com:

Patch Set 2: LUCI-TryBot-Result+1

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Message from Dmitri Shuralyov:

Patch Set 3: Auto-Submit+1

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Message from Dmitri Shuralyov:

Patch Set 3: Code-Review+1

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This PR is being closed because golang.org/cl/784400 has been merged.