gcc: Allow using libc++

gcc: Allow using libc++

With the addition of the C++ runtime setting added recently, allow
gcc to use libc++ as its runtime. There's some minor fixes still
required, such as allowing setting the unwinder library. But this
allows for testing libc++ with gcc.

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>