Description
newlib for the Zephyr SDK is currently compiled with __SINGLE_THREAD__ defined, and this causes most locking functions, including retargetable locking, to be omitted.
__SINGLE_THREAD__ is defined for Zephyr SDK newlib build because CT_LIBC_NEWLIB_MULTITHREAD is not explicitly set to y, and crosstool-ng consequently specifies --disable-newlib-multithread during newlib configure (note that this option is enabled by default unless --disable-newlib-multithread or --enable-newlib-multithread=no is specified).
In order to properly support newlib multithreading in Zephyr, we must build newlib without __SINGLE_THREAD__ defined.
Also note that GNU ARM Embedded builds without __SINGLE_THREAD__ despite not having --enable-newlib-multithread specified because this option is, as previously mentioned, enabled by default.