updated asap7 platform config.mk to support multi vt

[jeffng-or]

• Updated jpeg_lvt to leverage ASAP7_USE_VT instead of manually adding the LVT asap7 lib files - more in line with aes_lvt and ethmac_lvt
• Extended ASAP7_USE_VT to support one or more Vt's.
• Default remains a single Vt (RVT)
• The first Vt specified is treated as the primary Vt and is used to set the tie hi/tie lo and other cells
• The other Vts are treated as secondary and their Lib, LEF, GDS, and fill cells are added to the appropriate lists
• Note that designs that use CCS will not be able to use multi-Vt unless we add the LVT and SLVT CCS files to the repo

[maliberty]

Are we using 6T cells?

@povik I've forgotten why we need this workaround but it would be nice if we didn't. Do you know/recall the history?

[povik]

I can see it was added in #1447 for use in the EQY check. If we switch to use Liberty models as discussed in #3179 we won't need this anymore.

[jeffng-or]

For the 6T question, it doesn't look like it. We only have only using 7.5T LEF/Lib/GDS in our repo.

I ended up duplicating what was already there for RVT. but can remove all the 6T files as part of this PR.