cmake: Toolchain abstraction: move LINKERFLAGPREFIX under GNU ld

* LINKERFLAGPREFIX's value is GNU ld specific, and
* LINKERFLAGPREFIX is not a convention always honored by all linkers.

Thus we shall not set it from the common root CMakeList.txt.
So we move to linker/ld/target.cmake.

No functional change expected.

This is motivated by the wish to abstract Zephyr's usage of toolchains,
permitting non-intrusive porting to other (commercial) toolchains.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>