arm float: Update compiler options

Apply the correct compiler options when building for a Cortex-M4
processor that has floating point enabled. Using 'softfp' allows
floating point to be used with existing libraries supplied by the
Zephyr SDK. The 'hard' option would require new libraries to be
shipped with the SDK.

Change-Id: I141c20f54c2241134510888688637930c7b560a2
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>