arch/mips: add option for toolchains supporting -mfpxx

arch/mips: add option for toolchains supporting -mfpxx

-mfpxx option was added in gcc-5.1.0 so make sure that users cannot
select the "xx" fp32 mode when using toolchains that have a gcc older
than 5.1.0.

-mfp32 and -mfp64 were added in gcc-4.1.0, so given the older gcc
version we support in Buildroot (in the GCC_AT_LEAST options) is 4.3 we
don't need to do anything else for them.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>