cmake/target_arm: Use NO_SPLIT for TOOLCHAIN_LD_FLAGS

The whole set of architecture flags must be specified together as
they might not make sense in isolation, e.g. -mfloat-abi=hard requires
a -mcpu value that might have an FPU. Use the NO_SPLIT feature to
bind all of the linker options together so that the linker can
compute the correct linker paths for toolchain-provided libraries like
libc and libgcc.

Signed-off-by: Keith Packard <keithp@keithp.com>