libcxx: Add package

Currently in OpenWrt, there are two libc++: libstdcpp and uClibc++. The
former is huge and the latter supports only C++98 with some basic support
for C++11. Those C++ versions seem to be specific to the compiler version

libcxx supports C++11 and above while being much smaller than libstdcpp.
On mt7621, these are the sizes of the ipks that I get:

libstdcpp: 460786
libcxx: 182881
uClibc++:67720

libcxx is faster than uClibc++ and is under active development as part of
the LLVM project while uClibc++ is effectively dead.

This PR modifies uclibc++.mk to expose the make menuconfig option. Further
cleanup is beyond the scope of this PR. What that means is, this is not
used by default.

A g++-libcxx wrapper based on the uClibc++ one was added. Works the same
way.

Compile tested with all packages that use uclibc++.mk in their Makefiles
under mipsel_24kc. kismet fails compilation but that package needs to be
cleaned up and updated.

Runtime tested with gddrescue, gdisk, dcwapd, bonnie++, and aircrack-ng
on a TP-Link Archer C7v2.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

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