-mnan option was added in gcc-4.9.0 so make sure that users cannot
select the NaN mode when using toolchains that have a gcc older
than 4.9.0, and also make sure that the -mnan option is not passed at
all to the toolchain-wrapper and target cflags.
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>