Remove __attribute__malloc__ from MYSWAP functions

Remove __attribute__malloc__ from MYSWAP functions

These functions are only used when the native sockets functions are not
available, e.g. when building miniperl on Windows following commit
19253ae62c, so gcc's warning about ignoring the __malloc__ attribute here
is not normally seen.

The addition of "a" to these functions in embed.fnc by
commit f54cb97a39 was presumably wrong since none of them actually
allocate any memory (nor did so at the time), so change it to just "R"
(which is implied by the "a" and is still appropriate).