toolchain-external: drop no longer needed CC_TARGET_<foo>_ variables

toolchain-external: drop no longer needed CC_TARGET_<foo>_ variables

Since the introduction of the GCC_TARGET_<foo> variables in
arch/arch.mk in commit bd0640a2139119e2fdb4b384ebf32d1edcb0fdaa
("arch: allow GCC target options to be optionally overwritten") and
the removal of the BR2_GCC_TARGET_CPU_REVISION, the CC_TARGET_<foo>_
variables in pkg-toolchain-external.mk map 1:1 with the corresponding
GCC_TARGET_<foo> variables.

So let's drop the CC_TARGET_<foo>_ variables, and use directly the
GCC_TARGET_<foo> ones.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>