[libc][bazel] add file printf targets and support

[libc][bazel] add file printf targets and support

This patch adds targets for printf and fprintf to the bazel build.
Additionally, it adds support for the build system to specify where
files should be written for testing purposes. This was necessary to
enable the fprintf test under bazel.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D147008