apply appropriate CFLAGS while generating symmap

When floating-point is enabled with GNU Tools for ARM Embedded
Processors [*], we got the following error:

  arm-none-eabi-ld: error: build/discoveryf4/f9.elf uses VFP register
  arguments, build/discoveryf4/f9.symmap.o does not

It results from unmatched compilation options while generating symmap.

[*] https://launchpad.net/gcc-arm-embedded