The macro my_offsetof() performs pointer arithmetics that may be
undefined behavior. As reported in MDEV-26272, it may cause
clang -fsanitize=undefined to generate invalid memory references.

struct PFS_events_statements: Convert to std::is_standard_layout
by encapsulating the standard-layout struct PFS_events instead of
deriving from it, so that the standard macro offsetof() can be used.

PFS_events_statements::copy(): Renamed from copy_events_statements().
A cast to void* is now needed in memcpy() to avoid GCC -Wclass-memaccess
"writing to an object ... leaves 64 bytes unchanged".