posix: Fix name collision with __bswap

__bswap_ in zephyr/sys/byteorder.h conflicts with __bswap_ in host's
byteswap.h. byteswap.h from host compiler used in posix_native_64 boards
causes a compilation issue.

This commit renames __bswap_ to BSWAP_ to prevent collision.

Before this commit a compilation error can be created by adding #include
<byteswap.h> to samples/net/sockets/echo/src/socket_echo.c

This does not change external API to byteorder.h, but does change
internal implementation which some other source files depend on.

Replaced manual byteswap operations in devmem_service.c with APIs from
byteorder.h which automatically converts to CPU endianess when necessary.

Fixes #44324

Signed-off-by: Jonathan Hamberg <jonathanhamberg@gmail.com>