[cleanup] export opnd.h to all of core/

Today arch/opnd.h is only manually imported, since it is rarely needed inside core/. However, we have some public headers that want its types and defines.

Today we have this in os_exports.h:

/* XXX: reg_id_t is not defined here, use ushort instead */
ushort
os_get_app_tls_base_offset(ushort /*reg_id_t*/ seg);
ushort
os_get_app_tls_reg_offset(ushort /*reg_id_t*/ seg);
void *
os_get_app_tls_base(dcontext_t *dcontext, ushort /*reg_id_t*/ seg);

New code for #2350 (closed) wants to use DR_NUM_GPR_REGS in a public header. We already have a separate internal define in arch.h NUM_GP_REGS.

I do not think there are any circular dependencies or other roadblocks to including opnd.h in arch_exports.h. The only cost would be build time.