Make libc++ the default STL.

Previously, code defaulted to using libstdc++ (which was GNU libstdc++
on the host, or Bionic's "libstdc++" on the target). Now we default to
libc++. The exception for this is when using mingw, because we cannot
build libc++ for mingw.

Change-Id: I68726fa26cdcb6263f17c838d3e859debd57fae1
Cleanspec for libc++ migration.

This affects most native code in the tree, so the only real option is
to nuke out from orbit.

Change-Id: Ia3a794731e703c3ca02b55f6f7827d3425dca5cb
Merge "Make libc++ the default STL."
Merge "Cleanspec for libc++ migration."