package/gauche: exclude build on archs not boehm gc ported

This patch limits gauch build only on architectures boehm gc is ported.

Will fix:
http://autobuild.buildroot.org/results/368/3681948013fcd39cc80b690b880924421e429c80//
http://autobuild.buildroot.org/results/ba7/ba7eb5a878ec25033596dc7c9b0f4678d4a83de5//
http://autobuild.buildroot.org/results/1cb/1cbf1ee5bde3d50b935ae8ba931c8d0cc1052acc//
http://autobuild.buildroot.org/results/8af/8afb36606eabdc6ae6e65a28a11877af39f9a7e0//



[Thomas:
 - add comment explaining where the architecture dependency is coming
   from, as suggested by Arnout.
 - move the definition of BR2_PACKAGE_GAUCHE_ARCH_SUPPORTS to the
   beginning of the Config.in file, to be consistent with what we do
   in other packages.]

Signed-off-by: Hiroshi Kawashima <kei-k@ca2.so-net.ne.jp>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>