cmake: Toolchain abstraction: introduce toolchain_ld_baremetal()

The intent of toolchain_ld_baremetal() is to collect the flags belonging
to non-hosted (i.e. POSIX-based) targets.

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>