gcc-runtime.inc: Add CPP support for x86-64-x32 tune

gcc-runtime.inc: Add CPP support for x86-64-x32 tune

Using the following setup (as specified in yocto sample code):

MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:libx32"
DEFAULTTUNE_virtclass-multilib-libx32 = "x86-64-x32"

We fail to compile simple CPP programs because CPP cannot
find relevant header files, looking for them in a non-existing place.
To fix this, we create a symlink of the name CPP expects and point it to
the corresponding existing directory.

[YOCTO#10354]
[YOCTO#10380]

Signed-off-by: Juro Bystricky <juro.bystricky@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>