sys/_offsetof.h: pacify GCC 12's macro redefinition check

GCC 12 has a strict definition of identity (all characters including
whitespace must be identical) when comparing "redefined" macros. Make
our definition exactly match the stddef.h that comes with GCC to fix
test-includes of netlink/netlink_snl.h. (Note: later versions of GCC
are a bit more flexible and don't have this problem.)

Reported by: des
Sponsored by: DARPA, AFRL
Fixes: 2adc3f0db187 ("stddef.h: centralize definition of offsetof()")