fesetround reports success on a target that ignores it

Found by the cross-target differential, 2026-09-27.

Resolution (2026-09-28)

EmitCFenvStubs (compiler/cparser.inc) gains real stubs: i386 sets MXCSR and the x87 control word (i386 keeps x87 for int64 conversions and Round); aarch64 FPCR and arm32 FPSCR, RMode [23:22], whose encoding swaps FE's up/down bits. riscv32 does its doubles in softfloat, which honours no mode, so its setter refuses (nonzero) any mode but FE_TONEAREST, as wasm32's fenv.c does. xtensa keeps the accept-and-ignore stub (no oracle here to check a change against). Fixture test/c_fesetround_rounds_or_refuses.c (1/3, -1/3 and 1e16+1 in each mode): v445 3 rows wrong per target, this build none.