sim/d10v: Use offsetof in a static assertion about structure layout.

clang 11 fails to compile the static assertion as it cannot compute
the pointer value at a compile time:

gdb/sim/d10v/interp.c:1149:37: error: static_assert expression is not an integral constant expression
  static_assert ((uintptr_t) &State == (uintptr_t) &State.regs,
                 ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

Instead, assert that the offset of State.regs is 0.

sim/d10v/ChangeLog:

	* interp.c (sim_create_inferior): Use offsetof in static
	assertion.