toolchain/buildroot: glibc requires kernel headers >= 4.5 with NaN-2008

toolchain/buildroot: glibc requires kernel headers >= 4.5 with NaN-2008

From sysdeps/unix/sysv/linux/mips/configure.ac in glibc:

if test -z "$arch_minimum_kernel"; then
  if test x$libc_cv_mips_nan2008 = xyes; then
    arch_minimum_kernel=4.5.0
  fi
fi

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