ndk: Add printf() et al to Android support library.

This adds a musl-based printf() implementation to the Android support
library. This adds support for exotic formatters like %a (hexadecimal
floats), required by the libc++ unit test suite, as found by Nico
Weber (thakis@chromium.org)

+ Add missing implementation of frexp()/frexpf()/frexpl()
  (required by vfprintf.c)

+ Minimize the differences between our vfwprintf.c implementation
  and the original musl one, by moving the output wrapper structure
  to src/stdio/stdio_impl.[hc].

BUG=36496

Change-Id: I1b8e166646f5680434e3899bbecdf9492141e4ab
14 files changed