pkg-toolchain-external.mk: fix s/CC_TARGET_ARCH/GCC_TARGET_ARCH/ typo

commit e0d14fb21b (toolchain-external: drop no longer needed
CC_TARGET_<foo>_ variables) dropped the CC_TARGET_* variables, but missed
one.  Fix that.

Reported-by: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
1 file changed