bpo-41617: Fix pycore_byteswap.h to support clang 3.0 (GH-22042) (GH-22044)

__builtin_bswap16() is not available in LLVM clang 3.0.

(cherry picked from commit e6905e4c82cc05897dc1bf5ab2b5b94b2b043a7f)
2 files changed