When using the OpenWrt toolchain as an external toolchain the build
failed due to missing LTO support. By choosing the GCC wrappers of
the tools this commit makes sure that the LTO-enabled executables
are being used.

Signed-off-by: Vincent Wiemann <vincent.wiemann@ironai.com>
[ wrap the commit description to 72 char ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>