ref-manual: Define TC_CXX_RUNTIME

This variable is used to denote the C/C++ compiler runtime to use.
Right now there are few alternatives out of the core which could
use this variable to define this property, the values it can take
are 'gnu' for gcc runtime. 'llvm' for using compiler-rt+libc++ and
'android' to use android runtime. Default settings is to use gnu
which is current silent default also.

(From yocto-docs rev: 9094510e08b3a7b215b0d3ebecf66475a57b4c7e)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -7972,6 +7972,12 @@ system and gives an overview of their function and contents.
       toolchain. You can use ``meta-sourcery`` as a template for adding
       support for other external toolchains.
 
+   :term:`TC_CXX_RUNTIME`
+      Specifies the C/C++ STL and runtime variant to use during
+      the build process. Default value is 'gnu'
+
+      You can select "gnu", "llvm", or "android".
+
    :term:`TEMPLATECONF`
       Specifies the directory used by the build system to find templates
       from which to build the ``bblayers.conf`` and ``local.conf`` files.