Correctly round double precision sqrt (#256)

As discussed in https://github.com/JuliaLang/julia/pull/43786, openlibm's sqrt function is incorrectly rounded for i387. IEEE requires correct rounding for these functions and LLVM relies on it. Fix that by setting the precision in the FPU control word (see e.g. e_ceil.S for similar FPU modifications).

i387/e_sqrt.S

@@ -8,12 +8,29 @@
 //__FBSDID("$FreeBSD: src/lib/msun/i387/e_sqrt.S,v 1.10 2011/01/07 16:13:12 kib Exp $")
 
 ENTRY(sqrt)
-	fldl	4(%esp)
+	pushl   %ebp
+	movl	%esp,%ebp
+	subl	$8,%esp
+
+	fstcw	-4(%ebp)		/* store fpu control word */
+	movw    -4(%ebp),%dx
+	andw    $0xfeff,%dx 	/* Set precision field to 64 bits (53 bit mantissa).
+	                           We assume it's set to 0b11 (extended precision),
+							   so zeroing out the low bit of the precision field,
+							   will correctly set the precision */
+	movw	%dx,-8(%ebp)
+	fldcw	-8(%ebp)		/* load modfied control word */
+
+	fldl	8(%ebp)
 	fsqrt
+
+	fldcw	-4(%ebp)		/* restore original control word */
+
+	leave
 	ret
 END(sqrt)
 
-	
+
 /* Enable stack protection */
 #if defined(__ELF__)
 .section .note.GNU-stack,"",%progbits