jack2: fix NGREG build failures

jack2: fix NGREG build failures

This commit replaces the patch adding nios2 "support" to jack2 by a more
generic approach that solves the NGREG definition problem by using
proper checks in the waf script. This allows to remove the
architecture-specific compile time conditionals, and will solve build
failures on other architectures than just nios2.

Fixes:

  http://autobuild.buildroot.net/results/d15dc78cdd74b51b551512ce1ba88e4d61219e86/ (xtensa)
  http://autobuild.buildroot.net/results/8244e3fdda52d3f4f24d7199e8855c99a2a7b4b1/ (arc)
  http://autobuild.buildroot.net/results/badb9ec391f9f0438d593a2768f2f6c6a873c021/ (microblaze)

This commit has been built tested with all the external toolchains used
in the autobuilders.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>