[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.

Backport of Core PR11138

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

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