lib/libc: Mark picolibc as providing full C library support

Select FULL_LIBC_SUPPORTED when picolibc is available.

Add picolibc as a secondary default C library when REQUIRES_FULL_LIBC is
selected. This is necessary as tests gated on FULL_LIBC_SUPPORTED need to
be sure that a full C library will be selected -- if only picolibc is
available, those tests will need to select that.

This should permit use of a picolibc-only crosstool-ng toolchain in
testing.

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