lib: newlib: Define _ANSI_SOURCE

This commit updates the Newlib integration to define `_ANSI_SOURCE`
in order to prevent Newlib from defining POSIX primitives in its
headers when GNU dialect is used (`-std=gnu*`).

Newlib `features.h` defines `_DEFAULT_SOURCE` when `__STRICT_ANSI__`
is not defined by GCC (i.e. when `-std=gnu*`), which results in the
Newlib headers defining POSIX primitives that are in conflict with the
POSIX primitives defined by Zephyr.

Newlib must not define POSIX primitives unless the feature test macros
such as `_POSIX_SOURCE`, `_GNU_SOURCE` and `_DEFAULT_SOURCE` are
explicitly defined.

Note that `-std=gnu` does not imply `_GNU_SOURCE` or `_DEFAULT_SOURCE`
because it is only supposed to instruct the compiler to use the GNU C
language dialect (i.e. GNU C language extensions).

Refer to the GitHub issue #52739 for more details.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>