cmake: Toolchain abstraction: move PROPERTY_LINKER_SCRIPT_DEFINES

Move PROPERTY_LINKER_SCRIPT_DEFINES to toolchain_ld_base.

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>