fix mips/mips64 build for old compilers

gcc 4.8 does not support nan flag.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Rules.mak

@@ -404,8 +404,8 @@ ifeq ($(TARGET_ARCH),mips)
 	CPU_CFLAGS-$(CONFIG_MIPS_N64_ABI)+=-mabi=64
 	CPU_CFLAGS-$(CONFIG_MIPS_O32_ABI)+=-mabi=32
 	CPU_CFLAGS-$(CONFIG_MIPS_N32_ABI)+=-mabi=n32
-	CPU_CFLAGS-$(CONFIG_MIPS_NAN_LEGACY)+=-mnan=legacy
-	CPU_CFLAGS-$(CONFIG_MIPS_NAN_2008)+=-mnan=2008
+	CPU_CFLAGS-$(CONFIG_MIPS_NAN_LEGACY)+=$(call check_gcc,-mnan=legacy)
+	CPU_CFLAGS-$(CONFIG_MIPS_NAN_2008)+=$(call check_gcc,-mnan=2008)
 	CPU_LDFLAGS-y += $(CPU_CFLAGS)
 endif