math: move i386 sqrt to C with inline asm

src/math/i386/sqrt.c

+#include "libm.h"
+
+double sqrt(double x)
+{
+	union ldshape ux;
+	unsigned fpsr;
+	__asm__ ("fsqrt; fnstsw %%ax": "=t"(ux.f), "=a"(fpsr) : "0"(x));
+	if ((ux.i.m & 0x7ff) != 0x400)
+		return (double)ux.f;
+	/* Rounding to double would have encountered an exact halfway case.
+	   Adjust mantissa downwards if fsqrt rounded up, else upwards.
+	   (result of fsqrt could not have been exact) */
+	ux.i.m ^= (fpsr & 0x200) + 0x300;
+	return (double)ux.f;
+}

src/math/i386/sqrt.s

-.global sqrt
-.type sqrt,@function
-sqrt:	fldl 4(%esp)
-	fsqrt
-	fnstsw %ax
-	sub $12,%esp
-	fld %st(0)
-	fstpt (%esp)
-	mov (%esp),%ecx
-	and $0x7ff,%ecx
-	cmp $0x400,%ecx
-	jnz 1f
-	and $0x200,%eax
-	sub $0x100,%eax
-	sub %eax,(%esp)
-	fstp %st(0)
-	fldt (%esp)
-1:	add $12,%esp
-	fstpl 4(%esp)
-	fldl 4(%esp)
-	ret