toolchain-scripts.bbclass: only install all MULTILIB_VARIANTS for image

toolchain-scripts.bbclass: only install all MULTILIB_VARIANTS for image

Install all MULTILIB_VARIANTS to sysroot and pack them only makes sense to
image recipe, so limit it to image recipe only, and it can fix:
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "core2-32"

$ bitbake lib32-meta-ide-support
WARNING: lib32-meta-ide-support-1.0-r3 do_configure: Manifest for ncurses not found, searched manifests:
/workspace2/lyang1/test_mlwarn/tmp-glibc/sstate-control/manifest-qemux86_64-ncurses.populate_sysroot
/workspace2/lyang1/test_mlwarn/tmp-glibc/sstate-control/manifest-core2-32-ncurses.populate_sysroot
/workspace2/lyang1/test_mlwarn/tmp-glibc/sstate-control/manifest-i586-ncurses.populate_sysroot
/workspace2/lyang1/test_mlwarn/tmp-glibc/sstate-control/manifest-x86-ncurses.populate_sysroot
/workspace2/lyang1/test_mlwarn/tmp-glibc/sstate-control/manifest-allarch-ncurses.populate_sysroot

This is because core2-64 is not in lib32-meta-ide-support's
PACKAGE_ARCHS which doesn't like image recipe (lib32-image recipe's
DEFAULTTUNE is not overridded)

[YOCTO #12298]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>