lm4f: Compile with FPU support

Specify the -mfloat-abi=hard and -mfpu=fpv4-sp-d16 flags so that FPU
is enabled by default.
While this compiles with FPU support, in order to use the floating point types,
the FPU must be explicitly enabled at runtime.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>