cmake: Toolchain abstraction: use LINKER, introduce toolchain_ld_base

toolchain_ld_base() represents flags that are fundamental to linking or
otherwise does not belong in any further specified linker flag category.

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>