runtime/linux/386: set FPU to 64-bit precision

Fixes #550.

R=r
CC=golang-dev
https://golang.org/cl/3469044

src/pkg/runtime/linux/386/rt0.s

@@ -5,5 +5,13 @@
 // Darwin and Linux use the same linkage to main
 
 TEXT _rt0_386_linux(SB),7,$0
+	// Linux starts the FPU in extended double precision.
+	// Other operating systems use double precision.
+	// Change to double precision to match them,
+	// and to match other hardware that only has double.
+	PUSHL $0x27F
+	FLDCW	0(SP)
+	POPL AX
+
 	JMP	_rt0_386(SB)