rockchip: Clean up M0 Makefile, clarify float-abi

rockchip: Clean up M0 Makefile, clarify float-abi

This patch shuffles the M0 Makefile flags around a bit trying to make
their purpose clearer and remove duplication. Since all three build
steps (compiling, assembling, linking) actually call GCC, remove the
misleading aliases $(AS) and $(LD) to avoid confusion that those tools
might be called directly. Split flags into a common group that has
meaning for all three steps and separate variables specific to each
step. Remove -nostartfiles which is a strict subset of -nostdlib.

Also add explicit parameters for -mfloat-abi=soft, -fomit-frame-pointer
and -fno-common. If omitted these settings depend on the toolchain's
built-in default and cause various problems if they resolve to
unexpected values.

Signed-off-by: Julius Werner <jwerner@chromium.org>