Fix TRAX kernel Makefile

[ChaseParate]

Fixes a few issues:

• Marks the Makefile target names under .PHONY to avoid make thinking the targets were already built.
  • The riscv target has the same name as the riscv directory, so make would think the target was already built, as the directory already exists (and thus not rebuild the kernel if necessary)
• Removes the checked in TRAX kernel/objdump output from the repository.
• Builds will fail if the riscv directory isn't present (wasn't an issue when the built kernels were checked into the repo).
  • I accomplished this by adding mkdir calls into the Makefile, but this can also be achieved by adding a .gitkeep file or something of the sort in that directory. I'm open to either.