newlib: add bbappend

Build newlib library to be thread-safe in multithreaded environment.

zephyrproject-rtos/zephyr#21518
zephyrproject-rtos/zephyr#21519
zephyrproject-rtos/zephyr#36201

https://sourceware.org/legacy-ml/newlib/2016/msg01165.html
https://sourceware.org/git/?p=newlib-cygwin.git;a=commit;h=bd54749095ee45d7136b6e7c8a1e5218749c87b6

Error log:
newlib/libc-hooks.c:310:1: note: in expansion of macro 'BUILD_ASSERT'
BUILD_ASSERT(IS_ENABLED(_RETARGETABLE_LOCKING), "Retargetable locking must be enabled");

Signed-off-by: Naveen Saini <[email protected]>
Tested-by: Jon Mason <[email protected]>

Author: saininav

meta-zephyr-core/recipes-core/newlib/newlib_%.bbappend

@@ -0,0 +1,5 @@
+# BUILD_ASSERT(IS_ENABLED(_RETARGETABLE_LOCKING), "Retargetable locking must be enabled");
+
+EXTRA_OECONF:append = " \
+        --enable-newlib-retargetable-locking \
+        "