nvmm: Fix building nvmm into kernel

nvmm: Fix building nvmm into kernel

* Remove 'DIAGNOSTIC' define from 'nvmm_os.h'.  First, 'DIAGNOSTIC' and
  'INVARIANTS' are different options.  Secondly, 'DIAGNOSTIC' should be
  defined in kernel configs, e.g., LINT64 defines it.
* Remove 'LOCORE' define from 'nvmm_x86_{svmfunc,vmxfunc}.S'.  It's
  already defined on the command line, when compiling ASM source files
  with the kernel (i.e., building the module into the kernel).
* Manually define 'LOCORE' for 'nvmm_x86_{svmfunc,vmxfunc}.o'
  compilation in the Makefile, fixing building as a standalone module.

This fixes building nvmm into kernel, like LINT64.

Thanks swildner for reporting the issue and helping with the fix.