s390x: add single instruction fma and fmaf

These are available in the s390x baseline isa -march=z900.

src/math/s390x/fma.c

+#include <math.h>
+
+double fma(double x, double y, double z)
+{
+	__asm__ ("madbr %0, %1, %2" : "+f"(z) : "f"(x), "f"(y));
+	return z;
+}

src/math/s390x/fmaf.c

+#include <math.h>
+
+float fmaf(float x, float y, float z)
+{
+	__asm__ ("maebr %0, %1, %2" : "+f"(z) : "f"(x), "f"(y));
+	return z;
+}