oeqa/selftest: test that newlib can build

Add a test to set TCLIBC=newlib and build newlib/libgloss.

This is the absolute minimum test, but at least it exercises the build
of this package.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>