[MIPS] Don't abort the build process if '-msym32' isn't supported

-msym32 and previously the strategy to tell the compiler to generate
64-bit code but the assembler to put it into 32-bit ELF was initially
a hack to get around the lack of proper 64-bit binutils support and
later turned into a neat optimization with significant code size
savings.  But it's really just an optimization so there is nothing
wrong with just dropping the option (and whatever else goes along with
it, I forgot all the nasty details) on the floor if due to a vintage
compiler it can't be suported.
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Makefile

@@ -602,7 +602,9 @@ ifdef CONFIG_64BIT
   endif
 
   ifeq ($(KBUILD_SYM32), y)
-    cflags-y += -msym32 -DKBUILD_64BIT_SYM32
+    ifeq ($(call cc-option-yn,-msym32), y)
+      cflags-y += -msym32 -DKBUILD_64BIT_SYM32
+    endif
   endif
 endif