tools: disable linker --as-needed option.

tools: disable linker --as-needed option.

The Xen build system is not currently compatible with the --as-needed
linker option. The proper fix for this is turning out to be rather
invasive to the build system so simply disable for now with the
intention of revisiting for the 4.2 release.

The --no-as-needed option is available at least since binutils 2.15
(released in May 2004) and hence I think can be unconditionally relied
on.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reported-by: Nathan March <nathan@gt.net>
Tested-by: Nathan March <nathan@gt.net>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>