lib/libc: picolibc: select its source with Kconfig choice

Implement the selection of the picolibc source in a Kconfig
choice as they are mutually exclusive.

The following table represents the possible choices, where 'X'
means that `PICOLIBC_SUPPORTED=n` and is guarded in the parent
level as `PICOLIBC=n`, so we do not need to care.

Module  Toolchain    C++   |       Choice
  0         0         0    |          X
  0         0         1    |          X
  0         1         0    |    Toolchain only
  0         1         1    |    Toolchain only
  1         0         0    |      Module only
  1         0         1    |          X
  1         1         0    |   Toolchain/Module
  1         1         1    |    Toolchain only

The current implementation favors `PICOLIBC_USE_TOOLCHAIN` over
`PICOLIBC_USE_MODULE` whenever possible, that preference is
maintained in this implementation as well - the TOOLCHAIN
source will be the default choice when the toolchain supports
it or when the C++ is enabled. Otherwise, fallback to MODULE.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>