gcc-target: Fix libatomic dependency tracking issues

The --enable-dependency-tracking option was added to workaround build
issues in libatomic. This fixes that build problem properly and removes
the flag since the dependency tracking code appears to be full of races
which are much deeper and harder to fix.

As per the automake manual, dependency tracking is only useful and worth
the build performance cost if you are doing more than one compile of the same
source code which in most cases we are not so this is a good thing anyway.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>