automake: Add missing libtool tag for cppasm language

automake: Add missing libtool tag for cppasm language

This is highlighted when CC is passing flags like -fPIE
which are stripped by libtool when building shared libs
and replaced with -fPIC, this actually results in CC
not matching the CC computed from cmdline created for
compiling and as a result libtool heuristic to compute
tags auotmatically fails.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>