More efficient C3X implementation (bp #5668)

[mergify]

This is an automatic backport of pull request #5668 done by Mergify.

Cherry-pick of b9c943b has failed:

On branch mergify/bp/stable/0.16/pr-5668
Your branch is up to date with 'origin/stable/0.16'.

You are currently cherry-picking commit b9c943b17.
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Changes to be committed:
	modified:   qiskit/circuit/library/__init__.py
	modified:   qiskit/circuit/library/standard_gates/__init__.py
	modified:   qiskit/qasm/libs/qelib1.inc
	new file:   releasenotes/notes/improved-c3x-decomposition-aa812a0bc8280c7a.yaml
	modified:   test/python/circuit/test_controlled_gate.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   qiskit/circuit/library/standard_gates/x.py

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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