Use LIBC_PICOLIBC_CXA_ATEXIT to keep this name in the same 'namespace'
as the other picolibc config names.
Enable retargetable locking by default. This allows the few locks in
picolibc to be implemented by the execution environment, rather than
disabling the locking code.
Signed-off-by: Keith Packard <keithp@keithp.com>