Create symlinks for all supported arches (armhf, armv7, aarch64, x86_64,
x86), not only armhf and aarch64. Skip creating symlinks for the arch
that the package gets built for, so we can include x86_64 in the list
without conflicting with the ccache package from Alpine.

Link to ../../../bin/ccache instead of /usr/bin/ccache, so it works
properly with crossdirect (where we mount the native chroot to /native
in the foreign chroot).

Add symlinks for clang and clang++.