go/tools/builders: Pass `CFLAGS` to assembly sources compilation (#4559)

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**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

This PR passes C flags to the invocation of the assembly compiler when
CGO is enabled.

**Which issues(s) does this PR fix?**

Fixes #4558

**Other notes for review**

* Please refer to the issue for a reproduction case.
* This PR includes a test case that directly exercises the fix, but does
not attempt to reconstruct the convoluted setup that originally exposed
the bug.