cmake: Toolchain abstraction: introduce LINKER variable

The LINKER variable is introduced to follow the same logic and flow as
the existing COMPILER variable: That is, each TOOLCHAIN is responsible
for choosing COMPILER and LINKER.

Currently, Zephyr's build system is hardcoded for GNU ld.
Reflect this in LINKER by letting all existing toolchains use GNU ld.

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>