[CRT:MATH] Import nextafter, nextafterf and nexttowardf from musl

Note: On Windows, where a long double is the same as a double, nextafterl is the same as nextafter, nexttowardl is the same as nexttoward.
Also nexttoward is the same as nextafter.

sdk/lib/crt/math/libm.h

@@ -0,0 +1,70 @@
+//
+// libm.h
+//
+//      Partly imported from musl libc
+//
+// Support header for musl math code.
+//
+// SPDX-License-Identifier: MIT
+//
+
+#pragma once
+
+#include <stdint.h>
+#include <float.h>
+#include <math.h>
+
+// Define _STATIC_ASSERT for compatibility with legacy CRT headers
+#ifndef _STATIC_ASSERT
+#define _STATIC_ASSERT(expr) extern char (*__static_assert__(void)) [(expr) ? 1 : -1]
+#endif
+
+// Make sure the sizes are correct
+_STATIC_ASSERT(sizeof(float) == 4);
+_STATIC_ASSERT(sizeof(double) == 8);
+_STATIC_ASSERT(sizeof(long double) == 8);
+
+_Check_return_ int __cdecl _isnanf(_In_ float _X);
+#define isnan _isnan
+#define isnanf _isnanf
+
+// musl has this in math.h
+static __inline uint32_t __FLOAT_BITS(float __f)
+{
+	union {float __f; uint32_t __i;} __u;
+	__u.__f = __f;
+	return __u.__i;
+}
+
+static __inline uint64_t __DOUBLE_BITS(double __f)
+{
+	union {double __f; uint64_t __i;} __u;
+	__u.__f = __f;
+	return __u.__i;
+}
+
+#define signbit(x) \
+	((sizeof(x) == sizeof(float)) ? (int)(__FLOAT_BITS(x)>>31) : \
+	                                (int)(__DOUBLE_BITS(x)>>63))
+
+static inline void fp_force_evalf(float x)
+{
+	volatile float y;
+	y = x;
+    (void)y;
+}
+
+static inline void fp_force_eval(double x)
+{
+	volatile double y;
+	y = x;
+    (void)y;
+}
+
+#define FORCE_EVAL(x) do {                    \
+	if (sizeof(x) == sizeof(float)) {         \
+		fp_force_evalf(x);                    \
+	} else {                                  \
+		fp_force_eval(x);                     \
+	}                                         \
+} while(0)