use the definition of FPdbleword to avoid having to
recreate system-dependent details about fp numbers

Reference: /n/atom/patch/applied2013/fabs
Date: Wed Jun 19 06:06:37 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/port/fabs.c	Wed Jun 19 06:06:10 2013
+++ /sys/src/libc/port/fabs.c	Wed Jun 19 06:06:10 2013
@@ -1,14 +1,11 @@
 #include <u.h>
 #include <libc.h>
 
-#define SIGN (1<<31)
-
 double
 fabs(double arg)
 {
-	FPdbleword x;
 
-	x.x = arg;
-	x.hi &= ~SIGN;
-	return x.x;
+	if(arg < 0)
+		return -arg;
+	return arg;
 }