toolchain: fix multilib symlink for external toolchains

ARCH_SUBDIR is a shell variable, so it should be referenced with
$${ARCH_SUBDIR}. Without this, no symbolic link is created, and the
external toolchain fails to work if the non-default multilib variant
is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>