gcc: When cross compiling a target gcc, target flags may be used on the host

gcc: When cross compiling a target gcc, target flags may be used on the host

The original fix worked some of the time, but not on all machines.  Fix
this by applying the change in a different way.  Following the example of
the existing BUILD_CFLAGS.

Below is the commit message from the original change to help explain why
this is needed:

Configure identifies a number of warning flags (WARN_CFLAGS and
WARN_CXXFLAGS) from the $CC value.  The cross compiler may be different
from the host compiler and may not support the same set of flags.  This
leads to problems such as:

cc1plus: error: unrecognized command line option "-Wno-narrowing"
cc1plus: error: unrecognized command line option "-Wno-overlength-strings"

Work around this problem by removing the warning flags from the
BUILD_CXXFLAGS value, in a way similar to the BUILD_CFLAGS.

(From OE-Core rev: be21c6e8e4f810e826538337dac6e34ed96e1f6f)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>