Commit 3f1132ecd27186fa3f1a87ead6a0389802baccbb breaks cross-compilation
with uclibc on arm and m68k because host_os is set to
arm-buildroot-uclinux-uclibc or m68k-buildroot-uclinux-uclibc.
To fix this, assume that default is linux and remove "host_os not
supported" error.