Support Zygote and app forking

[k-sareen]

With the merging of https://github.com/mmtk/mmtk-core/commit/5ab62f96c006475285b00b6d20a8b1bf83b74a4d, we should be able to support the Zygote and app forking.

Things to do (in no particular order):

• [x] Implement support for boot images
• [x] Implement support for stopping all GC threads (including companion thread) in ART
• [x] Add Zygote space to CommonPlan
• [x] Add Zygote space to space mappings for plans
• [x] Implement a PreFirstZygoteForkCollection that compacts the Zygote space and freezes it
• [x] Need to have coherent status of Zygote across MMTk and ART (i.e. IsZygoteProcess() and HasZygoteSpace())
• [x] Full defrag GC just for PreFirstZygoteForkCollection
• [x] Support command-line and Zygote-mode at the same time
• [x] Pinning bits for non-moving allocations
• [x] Fix bugs
• [x] Enable nursery GCs after Zygote has been forked for the first time

[k-sareen]

Closed with: https://github.com/k-sareen/mmtk-art/commit/d9856448b96c511fa238832cb465911a902637bf https://github.com/k-sareen/art/commit/e60063c06162d006a919a0e8d493a668649f769a https://github.com/k-sareen/mmtk-core/commit/8295a82d3d159f2b5023db1c62ebc5e20585eea2