libc: Control Z_LIBC_PARTITION_EXISTS from Kconfig

Instead of adding every possible subsystem which places variables in the C
library memory partition in libc-hooks.h, place those conditions in the
related Kconfig files and simplify the libc-hooks.h to just looking at
CONFIG_NEED_LIBC_MEM_PARTITION.

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