amd64 kernel __storeload_barrier: quiet gcc -Warray-bounds

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.

Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45694