toolchain-external: fix registration of hook for Sourcery Aarch64 toolchain

toolchain-external: fix registration of hook for Sourcery Aarch64 toolchain

As noticed by Romain Naour, commit
4d39ca1c2ab19766abf0430abe9b65ba0e16602b ("toolchain-external: fix
installation for CodeSourcery AArch64 toolchain") has a small bug where
a post-install hook doing fixups in TARGET_DIR was registered as a
staging installation hook while it should have been registered as a
target installation hook. This commit fixes this inconsistency.

Reported-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>