This adds support for the retargetable locking implementation in
newlib 3. This feature will be enabled in the future toolchain builds.
With the present version of the toolchain, this code doesn't get used.
When _RETARGETABLE_LOCKING gets enabled, newlib locking implementation
will be modified as follows:
- Legacy ESP-specific _lock_xxx functions are preserved. This is done
because ROM copies of newlib in ESP32 and ESP32-S2 rely on these
functions through the function pointer table. Also there is some
code in IDF which still uses these locking functions.
- New __retarget_lock_xxx functions are introduced. Newlib expects
these functions to be provided by the system. These functions work
pretty much the same way as the ESP-specific _lock_xxx functions,
except one major difference: _lock_acquire receives the lock pointer
by value, and as such doesn't support lazy initialization.
- Static locks used by newlib are now explicitly initialized at
startup. Since it is unlikely that these static locks are used at
the same time, all compatible locks are set to point to the same
mutex. This saves a bit of RAM. Note that there are still many locks
not initialized statically, in particular those inside FILE
structures.
