cmd/gomobile: specify clang++ compiler for C++ compilation, setup CGO_CXXFLAGS

[tadovas]

Fixes #27508, #17343

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Patch Set 1:

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[tadovas]

Fixes golang/go#17343 and golang/go#27508

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Message from Gerrit User 7435:

Patch Set 1:

(2 comments)

Thank you. A few tweaks to the commit message and this can go in.

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Message from Gerrit User 12446:

Uploaded patch set 2: Commit message was updated.

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Message from Gerrit User 12446:

Uploaded patch set 3: Commit message was updated.

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Message from Gerrit User 29177:

Patch Set 3:

(2 comments)

Thanks for the tips. Updated

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Message from Gerrit User 7435:

Patch Set 3:

(1 comment)

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Message from Gerrit User 7435:

Patch Set 3:

Patch Set 3:

(1 comment)

"Nit", not "not". Phone corrected me.

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Message from Gerrit User 12446:

Uploaded patch set 4: Commit message was updated.

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Message from Gerrit User 29177:

Patch Set 4:

(1 comment)

Fixed that too :)

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This PR is being closed because golang.org/cl/134975 has been merged.