native.bbclass: Override TARGET_ flags too

TARGET_LDFLAGS is currently defined in bitbake.conf to contain
${TARGET_LINK_HASH_STYLE} which differs between MIPS and other
targets. Since TARGET_LDFLAGS is an exported variable it affects the hash
of every shell task even if it is not used.

We don't want native recipe tasks to have different hashes purely because
they happen to have been built in order to satisfy dependencies for
different MACHINEs since this causes lots of churn in the native sysroot
when switching between MACHINEs.

Making native.bbclass override TARGET_LDFLAGS to use BUILD_LDFLAGS ensures
consistent hashes and is a sensible thing to be doing anyway.

Although they don't appear to have the same detrimental affect on task
hashes TARGET_CPPFLAGS, TARGET_CFLAGS and TARGET_CXXFLAGS should be
overridden too.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>