arch/mips: inverse the NaN logic

arch/mips: inverse the NaN logic

Currently the possibility to choose the NaN encoding is conditional to
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 NaN-2008 are selected, we restrict the toolchain
selection to at least a gcc-4.9.

But now, the option with the NaN type is always set, so we must enclose
the code in gcc.mk inside a HAS_NAN_OPTION condition, as is already done
for the external toolchain case.

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>