According to the C standard, an offsetof expression must evaluate to an
address constant, otherwise it's undefined behavior.
Fixes https://bugs.dolphin-emu.org/issues/12409
See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942
There are still improper uses of offsetof (mostly in JitArm64) but
fixing that will take more effort since there's a PPCSTATE_OFF wrapper
macro that is sometimes used with non-array members and sometimes used
with arrays and variable indices... Let's keep that for another PR.