Replace M_PI2l with lit_pi_2_d in libm-test.inc

This is useful in situations where the long double type is
less precise than the type under test.  This adds a new
wrapper macro LITM(x) to each type to append the proper
suffix onto macro constants found in math.h.

math/test-double.h

@@ -26,4 +26,5 @@
 #define PREFIX DBL
 #define LIT(x) (x)
 #define TYPE_STR "double"
+#define LITM(x) x
 #define FTOSTR snprintf