cmd/gomobile: pass -a build flag to go build command

[jasikpark]

The gomobile build command wraps "go build" and parses many build
flags including "-a", but it was not actually passing the "-a" flag
through to the underlying go build command.

The "-a" flag forces a rebuild of packages and dependencies, which
is useful for ensuring clean builds when needed.

Add the missing code to pass the "-a" flag through to the go build
command when buildA is true, matching the pattern used for other
build flags like "-x" and "-gcflags".

Fixes golang/go/#76466

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Patch Set 1:

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[gopherbot]

This PR (HEAD: 92bbebf) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/mobile/+/724360.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
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  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
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• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[jasikpark]

golang/go#76466

[gopherbot]

Message from Caleb Jasik:

Patch Set 2:

(1 comment)

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[gopherbot]

Message from Hajime Hoshi:

Patch Set 5:

(2 comments)

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[gopherbot]

Message from Hajime Hoshi:

Patch Set 5:

(1 comment)

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