i386 kernel __storeload_barrier: quiet gcc -Warray-bounds

[rlibby]

Summary

Use a constant input operand instead of an output operand to tell the
compiler about OFFSETOF_MONITORBUF. If we tell it we are writing to
*(u_int *)OFFSETOF_MONITORBUF, it rightly complains, but we aren't.
The memory clobber already covers the necessary semantics for the
compiler.

Test Plan

No testing done yet! Just following D45694. Will build i386 GENERIC and disassemble membarrier_action_seqcst