Picolibc source was derived from stripped-down version of newlib
without GPL-related bits. At the moment, only parts of the test suite
are licensed under GPLv2. This patch eliminated non-specific LGPL/GPLv3
and kept COPYING.NEWLIB (without GPL'd parts) and GPLv2.

Signed-off-by: Jim Huang <jserv@biilabs.io>