package/pkg-cargo.mk: use CARGO_TARGET_<target>_LINKER instead of cargo config file

package/rustc/rustc.mk produces a config file in $(CARGO_HOME) that
points to the linker. We can simplify that by using the
CARGO_TARGET_<target>_LINKER environment variable.

This will also fix problems when host == target, where the target
linker gets picked up incorrectly.

Fixes:

  http://autobuild.buildroot.net/results/2183f1835f2ef553e45e83959910205127b2b259/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>