[PATCH] For asm n and similar constraints use maybe_const_value (PR c++/56302)

Hi! My SIZEOF_EXPR deferred folding changes regressed some sys/sdt.h macros at -O0, the sizeof in there is no longer folded, so instead of say n (-8) we ended up with n (-1 * (int) sizeof (void *)) and similar, and as at -O0 we don't really optimize, the asm got rejected. Fixed thusly,

Re: [PATCH] For asm n and similar constraints use maybe_const_value (PR c++/56302)

OK.