When compiling the kernel as a whole program it is possible that these functions may
be inlined and not emitted in the resulting binary. However at the same time the
compiler may itself emit calls to these functions. Marking these functions as
externally visible tells the compiler that there may be more usages of them than
it sees immediately in the source code, in this cases usages that the compiler
itself is going to generate
Some versions of GCC replace a struct copy with a call to 'memcpy',
which it is fully in its rights to do. Unfortunately on ARM platforms,
that have no direct calls to memcpy, the optimizer (in the presence
of -fwhole-program) drops the implementation of memcpy, even as it
is outputting calls to it.
Add the USED attribute prevents the optimizer from dropping the
function eagerly.
