arch/mips: inverse the mfpxx logic

arch/mips: inverse the mfpxx logic

Currently, the possibility to choose the floating point mode (32, xx or
64) is conditional on having a sufficiently recent gcc version.

Which means that the architecture selection depends on the gcc version.

But that's opposite to what we've always done in Buildroot: the software
versions are conditional to the architecture options. There is nothing
we can do about the hardware: it is there, we can't change it, while we
can restrict ourselves to using software that is working on said
hardware.

Thus, we inverse the logic, to move the condition onto the software
side: whenever mfpxx is selected, we restrict the toolchain selection to
at least a gcc-5.

And now, the blind BR2_TOOLCHAIN_HAS_MFPXX_OPTION symbol is no longer
needed, so we get rid of it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>