MIPS: Fix build with binutils 2.24.51+

Starting with version 2.24.51.20140728 MIPS binutils complain loudly
about mixing soft-float and hard-float object files, leading to this
build failure since GCC is invoked with "-msoft-float" on MIPS:

{standard input}: Warning: .gnu_attribute 4,3 requires `softfloat'
  LD      arch/mips/alchemy/common/built-in.o
mipsel-softfloat-linux-gnu-ld: Warning: arch/mips/alchemy/common/built-in.o
 uses -msoft-float (set by arch/mips/alchemy/common/prom.o),
 arch/mips/alchemy/common/sleeper.o uses -mhard-float

To fix this, we detect if GAS is new enough to support "-msoft-float" command
option, and if it does, we can let GCC pass it to GAS;  but then we also need
to sprinkle the files which make use of floating point registers with the
necessary ".set hardfloat" directives.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Cc: Matthew Fortune <Matthew.Fortune@imgtec.com>
Cc: Markos Chandras <Markos.Chandras@imgtec.com>
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/8355/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/fpregdef.h

@@ -14,6 +14,20 @@
 
 #include <asm/sgidefs.h>
 
+/*
+ * starting with binutils 2.24.51.20140729, MIPS binutils warn about mixing
+ * hardfloat and softfloat object files.  The kernel build uses soft-float by
+ * default, so we also need to pass -msoft-float along to GAS if it supports it.
+ * But this in turn causes assembler errors in files which access hardfloat
+ * registers.  We detect if GAS supports "-msoft-float" in the Makefile and
+ * explicitly put ".set hardfloat" where floating point registers are touched.
+ */
+#ifdef GAS_HAS_SET_HARDFLOAT
+#define SET_HARDFLOAT .set hardfloat
+#else
+#define SET_HARDFLOAT
+#endif
+
 #if _MIPS_SIM == _MIPS_SIM_ABI32
 
 /*