libc/picolibc: Support Picolibc for toolchains without bundled Picolibc

Allow Picolibc to get build from module, if the toolchain does not
include a bundled picolibc.
This should always be possible, except for native applications and
if libcpp is required.

Signed-off-by: David Schneider <david.schneider@chargepoint.com>