arm64/vmm: Define a dummy _start symbol in vmm_hyp_blob.elf

To silence a linker warning about _start being missing. This blob
contains code executed at EL2 and is only meant to be entered via
exception handlers.

Reviewed by: bz, emaste
Fixes: 47e073941f4e ("Import the kernel parts of bhyve/arm64")
Differential Revision: https://reviews.freebsd.org/D44735