cmake: toolchain: fix unsupported warnings for C++ builds

Compile with arm-none-eabi-g++ (10.2.0 with -std=c++17) generates a
warning for every C++ source file:
"cc1plus: warning: command-line option '-Wno-pointer-sign' is valid for
C/ObjC but not for C++"

Get rid of those warnings by adding '-Wno-pointer-sign' to the list of
excluded CXX options.

Signed-off-by: Rihards Skuja <rihardssk@mikrotik.com>
1 file changed