Stop building assym.o into the module.

DTrace includes assym.s, to build this we build assym.o, however
this is unneeded as assym.s only contains macros.

Remove the need to build this by removing it from OBJS, but keep assym.s
in the module dependencies via DPSRCS.

This fixes the build when there is no assembler, e.g. on arm64 without
the external binutils.

Submitted by: andrew
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10041