newlib: Drop _fstat implementation

We only need to implement fstat(), so drop the _fstat implementation and
have fstat() match the newlib docs for minimal implementation and only
set st_mode = S_IFCHR.

Change-Id: Iba9042707d1ee5975ab98293cfe20e2996b17c05
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>