Correctly detect hard/soft float ABI on arm32

The old code would detect -mfloat-abi=soft as softfp, but would treat
-mfloat-abi=softfp as hard float.

tools-for-build/arm-softfp-test.c

@@ -1,6 +1,17 @@
 int main ()
 {
-#if defined __arm__ && defined __SOFTFP__
+/*
+   The C compiler defines __ARM_PCS_VFP when the hard float ABI is in
+   use and floats/doubles are passed to and returned from functions
+   via VFP registers. Otherwise, floats are treated as integers for
+   the purpose of function calls.
+
+   Confusingly, __SOFTFP__ is defined only when software floating
+   point emulation is used within a function, and doesn't reveal which
+   function calling convention is used. It will be defined with
+   -mfloat-abi=soft but not -mfloat-abi=softfp
+*/
+#if defined __arm__ && !defined __ARM_PCS_VFP
     return 104;
 #else
     return 0;