Use context builddir for removing abandoned files

Removing abandoned files needs to know where the .ninja_log file is
stored.  Export the ninja builddir value from Context and use it to
determine the .ninja_log path in any stage.

The ninja builddir (where ninja stores its .ninja_log and .ninja_deps
files) and the bootstrap.BuildDir (where build output files are written)
are distinct, so to reduce confusion replace SetBuildDir with
SetNinjaBuildDir.