support -mfpmath=387 on x86_64

apparently somebody wants this for something... and it doesn't hurt.

arch/x86_64/bits/float.h

@@ -1,5 +1,9 @@
 #define FLT_ROUNDS 1
+#ifdef __FLT_EVAL_METHOD__
+#define FLT_EVAL_METHOD __FLT_EVAL_METHOD__
+#else
 #define FLT_EVAL_METHOD 0
+#endif
 
 #define LDBL_MIN     3.3621031431120935063e-4932L
 #define LDBL_MAX     1.1897314953572317650e+4932L