Expose active and available platform lists via SBDebugger API

Summary:
The available platform list was previously only accessible via the
`platform list` command, this patch makes it possible to access that
list via the SBDebugger API. The active platform list has likewise
been exposed via the SBDebugger API.

Differential Revision: https://reviews.llvm.org/D35760

llvm-svn: 310452