TF-A implementation of `memcpy_s()` does not comply with C11 standard.

[sandrine-bailleux-arm]

memcpy_s() function got introduced in the C11 standard. TF-A libc provides an implementation of it in file lib/libc/memcpy_s.c.

This implementation does not fully comply with the C11 standard for the following reasons:

1. (dsize > ssize) is allowed by the standard and thus should not be treated as an error. In this case, memcpy_s() should just copy ssize bytes.
   Note that (ssize > dsize), on the other hand, is a genuine error case and TF-A code is correct for this one. It matches the following sentence from the C11 standard:

n shall not be greater than s1max

2. The following behaviour from the C11 standard is not implemented:

If there is a runtime-constraint violation, the memcpy_s function stores zeros in the first s1max characters of the object pointed to by s1 if s1 is not a null pointer and s1max is not greater than RSIZE_MAX.

3. I don't see anything in the C11 standard that forbids ssize to be zero. It just would not copy any byte at all but it should not be treated as an error.