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>

toolchain/toolchain-buildroot/Config.in

@@ -52,6 +52,7 @@ config BR2_TOOLCHAIN_BUILDROOT_GLIBC
 	depends on !BR2_STATIC_LIBS
 	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_2
 	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10 || !BR2_powerpc64le
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_5 || !BR2_MIPS_NAN_2008
 	depends on !BR2_powerpc_SPE
 	select BR2_TOOLCHAIN_USES_GLIBC
 	# our glibc.mk enables RPC support