cmd/gomobile: allow arm64 host toolchain on Android (Termux)

[fann22]

cmd/gomobile: improve Android/arm64 host compatibility

Gomobile historically assumes that an arm64 host cannot provide an
arm64 NDK toolchain (due to official NDK limitations up to r23), and
therefore treats GOARCH=arm64 as unsupported. This causes gomobile
to fail immediately when run in Android/arm64 environments such as
Termux, even when a working arm64 host toolchain is available.

This change adds a minimal check to allow gomobile to use arm64 host
toolchains when GOOS=android and a Termux-style environment is
detected. The existing behavior for all other platforms remains
unchanged.

In addition, gomobile no longer overrides CC/CXX if the caller has
already provided them, allowing custom toolchains to be used safely.

This enables gomobile bind and gomobile build to run successfully
under Termux without affecting desktop builds.

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[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

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Next steps:
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[gopherbot]

Message from Hajime Hoshi:

Patch Set 1:

(2 comments)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/727260.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Hajime Hoshi:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/727260.
After addressing review feedback, remember to publish your drafts!