[ARM] Handle conflicts between -mfpu and -mfloat-abi options.

Summary: This patch implements warnings/downgradable errors for
invalid -mfpu, -mfloat-abi option combinations (e.g. -mfpu=none
-mfloat-abi=hard).

Change-Id: I94fa664e1bc0b5855ad835abd7a50a3e0395632d