Add Bionic's printf/scanf implementation files to the build.

The files were added to sources/android/support/src/stdio in the last
commit. This commit adds them to the build. This requires minor changes
to three of the c files, to make them build without the rest of Bionic.

With this commit, printf() (etc) will run the code in
sources/android/support/src/stdio, but the behavior of that function is
still identical to its behavior in Bionic.

Change-Id: I36800b0546877748d0ac6dcfb96ceb44258a6fa1