Create symbolic boot vdex files for different ISAs

Create only one vdex file and share between different ISAs.
By sharing vdex file, zygote32 and zygote64 can share the same file and
the system partition size is reduced by about 32MB.

Test: make framework & check boot.vdex and boot-*.vdex files
Test: test system boots with no selinux violations

Bug: 64211239

Change-Id: I31ede4b6aa342f2047d9309b8c631217e6699202