conf: add C++ flags for uninative interoperatility

conf: add C++ flags for uninative interoperatility

Create a common include file for compiler flags which allow native binaries to
be interoperable on a wide range of hosts. In particular the C++ ABI is
problematic so choose the CXX11 version to allow interoperation between gcc4 and
gcc5 based hosts. Moving this to a common include instead of uninative.bbclass
allows uninative to be configured later and used in the eSDK (where its
mandatory) even if the base configuration doesn't enable uninative by default
(e.g. nodistro in OE-Core).

[ YOCTO #10645 ]

(From OE-Core rev: 60c912ae9306532bdd4c5e09a65863ee77c12f43)

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