libc: picolibc: extend support beyond Zephyr SDK

The picolibc kconfigs were tied to ZEPHYR_TOOLCHAIN_VARIANT
and it only supported the Zephyr SDK, which is the only
toolchain having picolibc built-in at the moment. This
commit generalizes that to use TOOLCHAIN_HAS_PICOLIBC
cmake variable the same way as newlib: TOOLCHAIN_HAS_NEWLIB.
This provides the ability for other toolchains to declare
their support for picolibc.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>