toolchain-external: remove unused calculation of ARCH_SUBDIR
In TOOLCHAIN_EXTERNAL_INSTALL_TARGET_LIBS, ARCH_SUBDIR is calculated but not
used, and can thus be removed. Since SYSROOT_DIR is only used for the
calculation of ARCH_SUBDIR, it can be removed too.
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>