We redefine the top-level macros to our own in order to maintain
compatibility with compound initializers by wrapping the mid or low
level definitions from the OS header.
This allows, hopefully optimized, macros from the host OS's headers to
be used when building any hosted target obviating the need for
NEED_GENERIC_BYTESWAPS unless the target simply doesn't define its
own optimized versions (MIPS!).
Throw in some 64-bit swaps for completeness' sake; they generate no code
if not yet used anyway.
Change-Id: I21b384b55fea46833d01ea3cad1ad8952ea01a11
