math: move x87-family fabs functions to C with inline asm

src/math/i386/fabs.c

+#include <math.h>
+
+double fabs(double x)
+{
+	__asm__ ("fabs" : "+t"(x));
+	return x;
+}

src/math/i386/fabs.s

-.global fabs
-.type fabs,@function
-fabs:
-	fldl 4(%esp)
-	fabs
-	ret

src/math/i386/fabsf.c

+#include <math.h>
+
+float fabsf(float x)
+{
+	__asm__ ("fabs" : "+t"(x));
+	return x;
+}

src/math/i386/fabsf.s

-.global fabsf
-.type fabsf,@function
-fabsf:
-	flds 4(%esp)
-	fabs
-	ret

src/math/i386/fabsl.c

+#include <math.h>
+
+long double fabsl(long double x)
+{
+	__asm__ ("fabs" : "+t"(x));
+	return x;
+}

src/math/i386/fabsl.s

-.global fabsl
-.type fabsl,@function
-fabsl:
-	fldt 4(%esp)
-	fabs
-	ret

src/math/x86_64/fabsl.c

+#include <math.h>
+
+long double fabsl(long double x)
+{
+	__asm__ ("fabs" : "+t"(x));
+	return x;
+}

src/math/x86_64/fabsl.s

-.global fabsl
-.type fabsl,@function
-fabsl:
-	fldt 8(%rsp)
-	fabs
-	ret