[backport#11138] Compat: Simplify bswap_16 implementation
[backport#11138] Compat: Simplify bswap_16 implementation

Summary:
e40fa98 Simplify bswap_16 implementation (danra)

Pull request description:

  Simplify bswap_16 implementation on platforms which don't already have it defined.
  This has no effect on the generated assembly; it just simplifies the source code.

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Backport of Core [[https://github.com/bitcoin/bitcoin/pull/11138 | PR11138]]

Test Plan:
  ninja all check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7790

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