gcc-cross: argument list too long when do_install

gcc-cross: argument list too long when do_install

There would be an "Argument list too long" error when the length of
tmpdir is longer than 190, this is caused by:

headers=`echo $(PLUGIN_HEADERS) | tr ' ' '\012' | sort -u`

The PLUGIN_HEADERS is too long before sort, so the "echo" can't handle
it, use the $(sort list) of GNU make which can handle the too long list
would fix the problem, the header would be short enough after sorted.
The "tr ' ' '\012'" was used for translating the space to "\n", the
$(sort list) doesn't need this.

This doesn't impact the output, so it doesn't need the PR bump.

[YOCTO #2591]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>