runtime: fix a comment regarding default floating point precision

The expected precision setting for the x87 on Win32 is 53-bit
but MinGW resets the floating point unit to 64-bit.  Win32
object code generally expects values to be rounded to double,
not double extended, precision.

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/8175044

src/pkg/runtime/asm_386.s

@@ -108,7 +108,7 @@ TEXT runtime·breakpoint(SB),7,$0
 	RET
 
 TEXT runtime·asminit(SB),7,$0
-	// Linux, Windows start the FPU in extended double precision.
+	// Linux and MinGW start 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.