Fix build to not bogusly always rebuild vmm.ko.

Rename vmx_assym.s to vmx_assym.h to reflect that file's actual use
and update vmx_support.S's include to match. Add vmx_assym.h to the
SRCS to that it gets properly added to the dependency list. Add
vmx_support.S to SRCS as well, so it gets built and needs fewer
special-case goo. Remove now-redundant special-case goo. Finally,
vmx_genassym.o doesn't need to depend on a hand expanded ${_ILINKS}
explicitly, that's all taken care of by beforedepend.

With these items fixed, we no longer build vmm.ko every single time
through the modules on a KERNFAST build.

Sponsored by: Netflix


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273214 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f