lib: newlib: Add malloc lock
lib: newlib: Add malloc lock

This commit adds a lock implementation for the newlib heap memory
management functions (`malloc` and `free`).

The `__malloc_lock` and `__malloc_unlock` functions are called by the
newlib `malloc` and `free` functions to synchronise access to the heap
region.

Without this lock, making use of the `malloc` and `free` functions from
multiple threads will result in the corruption of the heap region.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr