libc: newlib: libc-hooks: Provide our own implementation of __chk_fail()

The version as shipped in Newlib itself is coded a bit sloppily for an
embedded environment. We thus want to override it (and make it weak, to
allow user apps to override it in turn, if needed). The desired
properties of the implementation are:

1. It should call _write() (Newlib implementation calls write()).
2. It should be minimal (Newlib implementation allocates message
on the stack, i.e. misses "static const").

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>