libc/picolibc: Make lock typing match between Zephyr and picolibc

Picolibc's retargetable locking is based upon having the user own the lock
type (struct __lock, along with typedef struct __lock *_LOCK_T), and then
having the picolibc internal code only refer to this type via the _LOCK_T
pointer typedef, leaving the actual struct undeclared there.

Zephyr wants to use 'struct k_mutex' for this type; the initial picolibc
port handled this by trying to redefine the picolibc locking to use 'void
*' instead of 'struct __lock *' by including '#define _LOCK_T void
*'. Which 'works' as long as the Zephyr code doesn't actually include
picolibc's sys/lock.h.

A recent picolibc change to support POSIX stdio locking has picolibc's
stdio.h including sys/lock.h, which breaks Zephyr's hack.

To fix this, create a real 'struct __lock' type as

	struct __lock { struct k_mutex m; };

Define all of the required picolibc locking API with this real type,
referring to the mutex inside without needing any casts.

This required switching the definition of the C library global lock from
K_MUTEX_DEFINE to the open-coded version, STRUCT_SECTION_ITERABLE_ALTERNATE
so that it has the correct type and still lands in the same elf section.

The only mildly inappropriate code left is that lock are allocated using
k_object_alloc(K_OBJ_MUTEX), which "works" because the size of 'struct
__lock` will exactly match the size of 'struct k_mutex' because of C's
struct allocation rules.

Signed-off-by: Keith Packard <keithp@keithp.com>