Work around GCC issue 99578

Work around GCC issue 99578

GCC 11.1 objects to pointers derived from a constant:

  error: '__builtin_memcpy' offset [0, 7] is out of the bounds [0, 0]  [-Werror=array-bounds]

This is a GCC bug, but work around it rather than turning array-bounds
checking off generally.

Reported-by: Damien Thenot <damien.thenot@vates.fr>
Reported-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Christopher Clark <christopher.w.clark@gmail.com>